Tire Size Question

06_Jetta_TDI

Member
Joined
Oct 21, 2005
Location
Highland, IL
TDI
Jetta TDI 06 Graphite Blue
I own an 06 Jetta TDI. I noticed that my Speedometer is 3 or 4 Miles off what it should be IE: If I'm going down the road 75 I'm really going just a little over 70. This is a fact that I am guessing many of you are familiar with. (VW even admited they knew about it)

Here is the question....

I have 205/55/16's on the TDI. Can anyone tell me if it would be a good or bad idea to go to a 205/65/16? This makes about a 3.9 mile an hour difference according to this tire size web site which would make me closer to correct on the speedometer and odometer, and would this not also show better fuel economy? (less revolutions per mile)
http://www.miata.net/garage/tirecalc.html

I have called a few places (including VW) and they said the only thing they can think of that would come into play would be the ABS.

Any thoughts?
 

PDJetta

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Nov 6, 2003
Location
Northern Virginia
TDI
'04 Jetta GLS TDI Pumpe Duce Platinum Grey w/ Leather
Not sure about the actual fuel economy. It may increase a little.

The speedometer purposfully reads 3 MPH high, but the odometer is close to dead on. You will have an odometer inaccuracy if you get the larger (in diameter) tires. It will record more miles than traveled, and thus your "calculated" fuel economy will show a large improvement due to the odometer inaccuracy.

--Nate
 

tditom

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Joined
Sep 5, 2001
Location
Jackson, MI
TDI
formerly: 2001 Golf GL, '97 Passat (RIP) '98 NB, '05 B5 sedan
its by design.

In my experience, larger diameter tires do not increase FE by any measurable amount.

For purposes of keeping track of FE accurately, as well as the maintenance schedule, I'd rather have the odo right on then the speedo.
 

Joe_Meehan

Top Post Dawg
Joined
Sep 3, 2005
Location
Ohio USA
TDI
NB TDI, 2002.5, Silver
PD is correct. All manufacturers know there will be variations in the speedometers. They all intentionally error optimistically to make sure you don't get a speeding ticket due to a slight error.

As for real mileage increase with the other tyres, it is difficult to say. The rolling resistance and the slight difference in gear ratio would both play into the question. I would expect whatever difference there was would be small or non-existent.
 

06_Jetta_TDI

Member
Joined
Oct 21, 2005
Location
Highland, IL
TDI
Jetta TDI 06 Graphite Blue
Well this kinda answered my question I guess. I'm not hearing anyone concerned with any ABS issues etc. I am a little concerned with the "rolling resistance and gear ratio" backfiring on me.

I know for a fact that in my case...my odometer IS off (I wonder if something happened with the newer Diesels? Why is my odometer off and yours is not?). I've done this several times....watching the mile markers get farther and farther away when I should be right on it....and my last trip to work should be 56.6 miles away and my odometer showed I traveled 57.4 miles.

I'm just having a hard time figuring out if I would make that big of a change from 205/55/16 to 205/65/16 if that would throw my odometer to far off in the other direction. Maybe I will go to a 205/60/16 for starters and see how that goes. I just want my speedometer/odometer to be more accurate than what it currently is. I have an F150 as a second vehicle and the odometer on it is dead on with the highway mile markers...
 

-=PYRO=-

Veteran Member
Joined
Dec 8, 2005
Location
Sackville Nova Scotia
TDI
2000 JET TDI
PDJetta said:
Not sure about the actual fuel economy. It may increase a little.

The speedometer purposfully reads 3 MPH high, but the odometer is close to dead on. You will have an odometer inaccuracy if you get the larger (in diameter) tires. It will record more miles than traveled, and thus your "calculated" fuel economy will show a large improvement due to the odometer inaccuracy.

--Nate
I believe this to be the opposite. A larger diameter tyre will have less revolutions than a smaller one thus less distance travelled recorded.
 

mrGutWrench

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Joined
Aug 29, 2002
Location
Carrboro, NC
TDI
'03 Jetta Wagon, 5-speed, 563K Miles (July '23)
06_Jetta_TDI said:
(snip) I know for a fact that in my case...my odometer IS off (snip)
__. I measured my '02 sedan (stock with Michelin MXV Energie tires) about 3 times; when those tires wore out about 87K miles, I went to the MXV-4+ tires; I measured those a couple of times too. I measured the odo against highway markers for 100 miles and compared the results. On my stock tires, the odo read about 98.7 miles for every 100 miles on the highway signs (this dropped slightly as the tires got worn). The replacement tires were more accurate - the odo on those showed 99.2-4 for every 100 miles on the highway markers.

__. I recently bought an '03 wagon. It had totally cr@p Japanese tires on it (too bad because it was in pretty good shape otherwise). Just this week, I put a set of 205/65/15* Michelin Energie's on it - I did a quick check on Wednesday - it appears that the odo shows about 96.4-6 for every 100 miles driven. I haven't checked the speedo yet but will do that this week.

(* Stock tires on the A4's with 15-inch wheels are 195/65/15's.)
 

Riles246

Active member
Joined
Feb 1, 2007
Location
Central Jersey
TDI
2002 Jetta TDI
If the speedometer is off, the odometer will be off by the same amount. They both read from the same sensor. Manufacturers don't "program in" an error- when VW or anybody else refers to an error, its simply because thats the way they roll off the assembly line.

Your first thought of changing tire size to correct the speedometer was right on the money. This is the cheapest and easiest way to correct a speedometer, assuming you were going to replace the tires anyway.

Mine is off as well by about the same as yours, and I will be changing the tire size to correct my speedometer/odometer pair as well.

ABS and all other functions will work just fine, as long as you change all four tires.
 

Slave2school

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Joined
Mar 20, 2004
Location
Angus, Ontario
TDI
99.5 used to at least...
Actually, Riles, the vw is made in this manner due to certain tire size laws in germany. Boils down to the manufacture must ensure that for th eapproved sizes of tires for the car the speedo never reads slower than the actual speed. My odometer is dead on, the speedo is apprx 3km/h fast depending on how fast you are driving, the error is not linear. the faster I go the error is up to around 5km/h at 195km/h.

Dakota digital sells a device to alter the VSS that can allow you to correct the speedometer if a person wants to do so with no tire size changes.
 

mrGutWrench

Top Post Dawg
Joined
Aug 29, 2002
Location
Carrboro, NC
TDI
'03 Jetta Wagon, 5-speed, 563K Miles (July '23)
mrGutWrench said:
__. I recently bought an '03 wagon. It had totally cr@p Japanese tires on it (too bad because it was in pretty good shape otherwise). Just this week, I put a set of 205/65/15* Michelin Energie's on it - I did a quick check on Wednesday - it appears that the odo shows about 96.4-6 for every 100 miles driven. I haven't checked the speedo yet but will do that this week.

(* Stock tires on the A4's with 15-inch wheels are 195/65/15's.)
__. Today, I checked the speedo. I set the cruise so that the speedo needle was touching the 60 mark (it was on the high side but still touching, that's the best I could do with setting the cruise) - I drove 10 miles based on road markers. It took 10 minutes and one second. I'm gonna call that pretty close to dead nuts.
 

doonboggle

Veteran Member
Joined
Jul 2, 2007
Location
Elgin, Texas
TDI
2006 Jetta w/Taktonic 6sp. transmission, Silver; 1981 Rabbit pick-up
My .02 cents worth

In reading many other postings during my educational period ... just got my Jetta ... I've read postings wherein the speedo/odo issue was 'talked' about in going both ways; both up and or down. This to me, seems to indicate that there is not an intentional 'programming' process by VW or any other manufacturer to build in a particular difference; one way or the other.

My spin on this, and is supported by the Highway Patrol of states having to calibrate all their new cars in order to determine the difference so that when they issue tickets the 'alleged' violation will be supported in court by an accurate observation, is that all vehicle speedometers and odos, no matter where manufactured, will have a certain degree of inaccuracy ... what with variations in tire manufacture, variations of the reading unit, and other minute variations here and there ... but when all are put together, account for a 2-3-4 mph difference.

My unit for example, in checking it the other day on a straightaway 5 mile posted calibration drive, came back with my speedo also off by 3-5 miles ... 60 visual - 55+ actual; and the odo off by a 1/10th in that 5 mile track (4.9 miles versus 5.0). So if I put smaller tires on to accomplish a more accurate speed, my total mileage accuracy is effected; and conversely, if larger tires to accomodate the mileage, the speedo is adversely affected.

So my solution is to drive 4 miles over the posted speed ... as needed; and to add 1/10th to my mileage measurements.

Works for me.........
 

silverbox

Veteran Member
Joined
Jul 24, 2005
Location
Halifax Nova Scotia
TDI
jetta wagon 2003 silver
Is there a way to reset in vagcom, so that the odometer reading is not changed when sizing up from 195 to 205
series tires?
 

Riles246

Active member
Joined
Feb 1, 2007
Location
Central Jersey
TDI
2002 Jetta TDI
Sorry to bring this up again, but there really is no way that a speedometer can be off but an odometer would be correct (unless the speedomoeter is programmed to be off, which I'll get to later). There is only one sensor in the car that sends absolute speed data- the VSS. There are 4 wheel speed sensors, but they only make relative readings to ensure that the tires are all spinning at the same speed (anti-lock brake sensors).

When the speedometer sensor is off, it will always be off by a percentage. So, if its 5 percent off, you'll see 105 mph at 100 mph, 52.5 mph at 50 mph, etc. etc. Or it could be off in the other direction. So, you usually won't notice that its off unless you use a gps at highway speeds.

This is why you can correct it by changing the size of your tires. If your speedometer is reading slow, you can get tires that have a smaller diameter. This will make your tires rotate more times each mile, which would result in a higher speedometer reading at the same speed. You just have to check your math to get it right on the money.

Now, on to the issue of the odometer and speedometer being programmed to be correct, and why this would be silly. If the odometer is correct, but the speedometer is off in order to allow for different tire sizes, then when the tires are actually changed to that different size, the speedometer would be right, but then the odometer would be wrong. Because remember- there is only one way for the car to know how fast it is going, and that is to count how many times the tires spin in a given amount of time. So why would a manufacturer plan on only having one instrument being correct at a given time?

So, if your odo is right (best checked by driving 100+ miles w/ a gps) and the speedo is wrong, then one will always be incorrect. You would then have to decide which one you want to be correct, and fix it through one of the many methods, tire size change being the easiest. If you make the speedo right, your odo will be off, and vice versa.

Feel free to correct me if I'm wrong, but until VW starts using GPS sensors for speed or mileage, tire size (and possibly final drive ratio) will always be the determinant in speedometer/odometer accuracy.
 

Slave2school

Top Post Dawg
Joined
Mar 20, 2004
Location
Angus, Ontario
TDI
99.5 used to at least...
The odometer is generally very accurate in these cars, the speedometer is not. Vag-com can show you the "true" speed which very closely matches the gps. The analog guage is set to read optomistic compared to what the "true" speed is that the computer "knows" you are driving. People with the climatronic can enter a hidden mode to show exactly what I am talking about without the use of vag-com. Yes, silly, but the way it is.
 

Ernie Rogers

Veteran Member
Joined
Oct 15, 2001
Location
Pleasant Grove, Utah
TDI
Beetle, 2003, silver
I did the tire change

Hi, guys,

I did the tire change we are talking about-- actually, the change from 205/55R16 to 205/60R16, in the Michelin MXV 4 tires. (I also changed from the "plus" to the S8 version.) I am considering going on up to the 205/65R16 in the newer Energy tires.

You get a respectable boost in mileage by increasing the tire diameter. Here are the diameters of these tires (going from 55% to 65% profiles, on 16 inch rims, right?)--

205/55R16........24.88 inches (These are calculated diameters)
205/65R16........26.49 inches

This is a diameter change of 6%. That means, say at 70 mph, that the engine will be turning 6% slower. There is also a 6% reduction in rolling resistance with increased diameter, all else being equal. For the Michelin Energy MXV4 Plus tires, the published rolling resistance coefficient is Crr=0.009. I'll just lower that by the ratio of diameters, to make it Crr=0.0085.

I have a fuel economy calculator in Excel that includes the TDI engine map (from Uponblocks, for pre-2004 engines). I can put in these changes in RPM and rolling resistance and tell you how the mileage changes. Here are the results--

205/55R16.........51.5 mpg at 70 mph
205/65R16.........53.5 mpg

As I said, I have the 60% profile now. This has made my speedometer almost dead on and my odometer reads 4.5% low. I am perfectly happy with this arrangement, and I won't be bothered by the further changes when I go on to the 65% profile.

Now, there's a little joke we need to discuss. The mileage improvement here is 5% from increasing the tire size. BUT the odometer reading will go down by 6%. So, if you go by the odometer, your mileage will appear to decrease by 1%, but that is just the appearance. Second part of the joke: All these calculations so far are based on actually going 70 mph. But, the speedometer has changed. If you ignore that and go by '70 mph indicated' before and after the tire change, there WILL BE a drop in actual fuel economy because you will be going 4.2 mph faster (that's 6%). Increasing speed that much should cut mileage by at least 5 mpg-- much more than the expected gain.

Ernie Rogers



PDJetta said:
Not sure about the actual fuel economy. It may increase a little.

The speedometer purposfully reads 3 MPH high, but the odometer is close to dead on. You will have an odometer inaccuracy if you get the larger (in diameter) tires. It will record more miles than traveled, and thus your "calculated" fuel economy will show a large improvement due to the odometer inaccuracy.

--Nate
 

Joe_Meehan

Top Post Dawg
Joined
Sep 3, 2005
Location
Ohio USA
TDI
NB TDI, 2002.5, Silver
Ernie Rogers said:
You get a respectable boost in mileage by increasing the tire diameter. Here are the diameters of these tires (going from 55% to 65% profiles, on 16 inch rims, right?)--...
Not exactly. You are making an assumption that your starting point is not optimized for efficiency. Often this is true. Gear ratios are chosen as compromises. Lower gives faster acceleration (in general) and higher gives better mileage. Going too far in either direction and you will loose on both counts.
 

hevster1

Vendor
Joined
Nov 14, 2005
Location
Columbia NJ
TDI
98 NB
-=PYRO=- said:
I believe this to be the opposite. A larger diameter tyre will have less revolutions than a smaller one thus less distance travelled recorded.
This is correct. Also this increases the faster you go as it is not linear it is a percentage. An increase in diameter causing mileage to increase depends on so many things such as tire weight, optimization, actual speed etc.
 

meganuke

TDIClub Contributor, Vendor
Joined
Mar 6, 2003
Location
VA/CT
TDI
2010 Ford Fusion Hybrid
205/60R16 tires makes my speedometer correct (verified by GPS at various speeds). I'm still trying to figure out how far off my odometer is.
 

doonboggle

Veteran Member
Joined
Jul 2, 2007
Location
Elgin, Texas
TDI
2006 Jetta w/Taktonic 6sp. transmission, Silver; 1981 Rabbit pick-up
Great information

And timely too. Was returning from 'big city' yesterday and while on nice long straightaway, took another reading of odo. At 8 miles, appears to be a slight increase of not quite 1/10th; based on the digital display. Speedo is, as before, off about 5 mph when compared with our Liberty; which is pretty much accurate when used in a measured 5 mile calibration straight flat stretch between Carson & Reno.

In coming thru the desert yesterday, and having verified my odo discrepancy, was thinking just how I could determine which tire size to go 'up' to to address the speedo adjustment.

Right now, with only 8k on car, these factory Turanza's ... same size you listed ... will have to last a while.

Lo & behold, your posting; which lays out the size for me and everything.

THANKS ! !:) :rolleyes: ;)



Ernie Rogers said:
Hi, guys,

I did the tire change we are talking about-- actually, the change from 205/55R16 to 205/60R16, in the Michelin MXV 4 tires. (I also changed from the "plus" to the S8 version.) I am considering going on up to the 205/65R16 in the newer Energy tires.

You get a respectable boost in mileage by increasing the tire diameter. Here are the diameters of these tires (going from 55% to 65% profiles, on 16 inch rims, right?)--

205/55R16........24.88 inches (These are calculated diameters)
205/65R16........26.49 inches

This is a diameter change of 6%. That means, say at 70 mph, that the engine will be turning 6% slower. There is also a 6% reduction in rolling resistance with increased diameter, all else being equal. For the Michelin Energy MXV4 Plus tires, the published rolling resistance coefficient is Crr=0.009. I'll just lower that by the ratio of diameters, to make it Crr=0.0085.

I have a fuel economy calculator in Excel that includes the TDI engine map (from Uponblocks, for pre-2004 engines). I can put in these changes in RPM and rolling resistance and tell you how the mileage changes. Here are the results--

205/55R16.........51.5 mpg at 70 mph
205/65R16.........53.5 mpg

As I said, I have the 60% profile now. This has made my speedometer almost dead on and my odometer reads 4.5% low. I am perfectly happy with this arrangement, and I won't be bothered by the further changes when I go on to the 65% profile.

Now, there's a little joke we need to discuss. The mileage improvement here is 5% from increasing the tire size. BUT the odometer reading will go down by 6%. So, if you go by the odometer, your mileage will appear to decrease by 1%, but that is just the appearance. Second part of the joke: All these calculations so far are based on actually going 70 mph. But, the speedometer has changed. If you ignore that and go by '70 mph indicated' before and after the tire change, there WILL BE a drop in actual fuel economy because you will be going 4.2 mph faster (that's 6%). Increasing speed that much should cut mileage by at least 5 mpg-- much more than the expected gain.

Ernie Rogers
 
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Blackbug1

Veteran Member
Joined
Oct 10, 2006
Location
Clifton Tn
TDI
2012 Jetta
I used my GPS to check my odometer on a 100 mile trip my odometer showed 97.9 miles. Scangauge turned over to 100 miles at 99.53 miles via GPS, at triple digits the scangauge does not show tenths. I have also thought about get taller tire to make my speedometer correct, altho I have correct scangauge and the speed is also right on. It is about time for tires, then my odometer will be off even more, but I guess I can use scangauge for the odometer reading. Thanks for the info Ernie good stuff
 

doonboggle

Veteran Member
Joined
Jul 2, 2007
Location
Elgin, Texas
TDI
2006 Jetta w/Taktonic 6sp. transmission, Silver; 1981 Rabbit pick-up
Now I am confused ... but what's new ....

I just re-thought my position; and came up with a problem. In reading Ernie's dialogue, I took it, in relation to my unit, that if my speedo is off by about 5 mph (speedo @60 ... actual @55), then his scenerio would address my situation. But in re-reading his post closer, it sounds like he went from the 55R tire to the 60R tire; which would exagerate the speedo problem for me even more.

Help me out here fellows....

If my speedo is shy by a negative 5 mph, then going to a larger tire would be wrong for me ... I'd need to increase revolutions rather than decrease ... which would then adjust my speedo to a higher reading ... which is what I'm looking for.

So if I have re-analyzed this correctly, I need a smaller radius tire; something less than the aforementioned 205/55R H version.

So, what should that be ... keeping in mind that I don't want to have it noticable ... if possible ... relative to the appearance within the fender well area.
Thanks:confused: :confused: :confused:




doonboggle said:
And timely too. Was returning from 'big city' yesterday and while on nice long straightaway, took another reading of odo. At 8 miles, appears to be a slight increase of not quite 1/10th; based on the digital display. Speedo is, as before, off about 5 mph when compared with our Liberty; which is pretty much accurate when used in a measured 5 mile calibration straight flat stretch between Carson & Reno.

In coming thru the desert yesterday, and having verified my odo discrepancy, was thinking just how I could determine which tire size to go 'up' to to address the speedo adjustment.

Right now, with only 8k on car, these factory Turanza's ... same size you listed ... will have to last a while.

Lo & behold, your posting; which lays out the size for me and everything.

THANKS ! !:) :rolleyes: ;)
 

Joe_Meehan

Top Post Dawg
Joined
Sep 3, 2005
Location
Ohio USA
TDI
NB TDI, 2002.5, Silver
Blackbug1 said:
I used my GPS to check my odometer
Something to watch out for is that GPS units don't all sample at the same rate. Some don't sample very often and if you are driving on curvy roads the GPS will tend to cut corners and under report the distance. I will have to check with my GPS pro and ask how hills might effect the results.
 

Ernie Rogers

Veteran Member
Joined
Oct 15, 2001
Location
Pleasant Grove, Utah
TDI
Beetle, 2003, silver
I get confused sometimes so bad I can't remember the problem--

Hi,

You said the speedo reads too high-- 60 when you are going 55.

Since the speedo gets its information from the engine speed, the engine is turning too fast, you need to slow it down-- to where the speedo only gets 55 mph.

Putting on bigger tires will slow down the engine and make the speedo more accurate.

Just doing the math, changing to the 205/65R16 tires should make your speedometer right on, BUT

I'm a little concerned about making mistakes. If you get the much bigger tires and go too far, you may be uncomfortable with the speedo reading TOO LOW. If the bigger tires interfere with the car structure, you'll have new tires you can't use.

I think you said this is just for thinking right now because your present tires are fairly new still. You have your answer-- bigger is better when you get to that point. I wonder, what can you do with the tires you have?

Have you tried increasing tire pressure? If tires are too soft, that might make the speedo read higher than it should.

Ernie Rogers

doonboggle said:
I just re-thought my position; and came up with a problem. In reading Ernie's dialogue, I took it, in relation to my unit, that if my speedo is off by about 5 mph (speedo @60 ... actual @55), then his scenerio would address my situation. But in re-reading his post closer, it sounds like he went from the 55R tire to the 60R tire; which would exagerate the speedo problem for me even more.

Help me out here fellows....

If my speedo is shy by a negative 5 mph, then going to a larger tire would be wrong for me ... I'd need to increase revolutions rather than decrease ... which would then adjust my speedo to a higher reading ... which is what I'm looking for.

So if I have re-analyzed this correctly, I need a smaller radius tire; something less than the aforementioned 205/55R H version.

So, what should that be ... keeping in mind that I don't want to have it noticable ... if possible ... relative to the appearance within the fender well area.
Thanks:confused: :confused: :confused:
 

doonboggle

Veteran Member
Joined
Jul 2, 2007
Location
Elgin, Texas
TDI
2006 Jetta w/Taktonic 6sp. transmission, Silver; 1981 Rabbit pick-up
Speedo versus actual

No...just the opposite. Speedo indicates 60 mph but actual is 55mph; as determined by clocking thru measured miles of 5 miles. Thus, it seems to me that if the tires revolutions dictate the speedo indication, that a smaller tire is appropriate; or am I misreading the symptoms?
Thanks

Yes, have increased the pressure. Since getting it a couple of weeks ago, in driving it home, it pulled to the right and in checking pressure, all were in the low to mid 20's. Plus, reading herein on forum about higher pressure for better mpg, raised the pressure to 40psi.







Ernie Rogers said:
Hi,

You said the speedo reads too high-- 60 when you are going 55.

Since the speedo gets its information from the engine speed, the engine is turning too fast, you need to slow it down-- to where the speedo only gets 55 mph.

Putting on bigger tires will slow down the engine and make the speedo more accurate.

Just doing the math, changing to the 205/65R16 tires should make your speedometer right on, BUT

I'm a little concerned about making mistakes. If you get the much bigger tires and go too far, you may be uncomfortable with the speedo reading TOO LOW. If the bigger tires interfere with the car structure, you'll have new tires you can't use.

I think you said this is just for thinking right now because your present tires are fairly new still. You have your answer-- bigger is better when you get to that point. I wonder, what can you do with the tires you have?

Have you tried increasing tire pressure? If tires are too soft, that might make the speedo read higher than it should.

Ernie Rogers
 
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Slave2school

Top Post Dawg
Joined
Mar 20, 2004
Location
Angus, Ontario
TDI
99.5 used to at least...
"Speedo indicates 60 mph but actual is 55mph;"

This means exactly what Rogers said. Your car needs larger tires so the engine spins slower, giveing you taller gearing, correcting the too fast speedometer.

You are backward in your assumption.
 

doonboggle

Veteran Member
Joined
Jul 2, 2007
Location
Elgin, Texas
TDI
2006 Jetta w/Taktonic 6sp. transmission, Silver; 1981 Rabbit pick-up
Okey-dokey

Slave2school said:
"Speedo indicates 60 mph but actual is 55mph;"

This means exactly what Rogers said. Your car needs larger tires so the engine spins slower, giveing you taller gearing, correcting the too fast speedometer.

You are backward in your assumption.


Ain't the first time............

Thanks.......
 

Slave2school

Top Post Dawg
Joined
Mar 20, 2004
Location
Angus, Ontario
TDI
99.5 used to at least...
It happens from time to time to the best of us when things don't look the way we think they should be. I do it all the time.
 

Pat Dolan

Veteran Member
Joined
Apr 19, 2002
Location
Martensville, SK
TDI
2003 A4 Variant, 2015 Q7
At highway speeds, rolling resistance isn't nearly as significant as aerodynamic drag. Since tire size changes don't change the overall resistance significantly, they won't/can't change the mileage.

Yes, you are slowing the engine every so slightly, but most of these beliefs are from the days of gassers. Slowing the engine while delivering the same power requires higher manifold pressure, thus considerably less pumping losses by the throttle plate. Diesels don't have a throttle (except, of course for some governed engines and the one in our EGR system), so they only benefit from the aerodynamic improvement of airflow through the engine, plus the infinitissimal change in rolling resistance and engine internal friction.

In other words, even with a fairly large change in RPM, you are not likely to find any significant change in mileage in a diesel. Yes, I have also collected tens of thousands of miles of data while doing auxiliary gearbox testing (on medium trucks) to verify the assumptions.
 

Ernie Rogers

Veteran Member
Joined
Oct 15, 2001
Location
Pleasant Grove, Utah
TDI
Beetle, 2003, silver
TDI engines get best mileage at low RPMs

Hi, Pat,

You could be right about those truck engines you tested. The typical TDI engine has a "sweet spot" from about 1200 RPM to 1800 RPM-- that's where you get the best efficiency. If you search on this web site for "Uponblocks" postings, and look for the engine map he posted, you can see for yourself.

I just finished doing a calculation, specifically looking at the effect of engine RPM (gear ratio) on mileage. Here are the results for a car going 60 mph, for a big change in gearing, of 40%--

Normal: 35 RPM/mph.........2100 RPM........60.2 mpg at 60 mph
Raised: 25 RPM/mph.........1250 RPM.........71.1 mpg

What you see here is just the effect of changing engine RPM with everything else about the car held constant. But, increasing tire size (with or without the gearing change) gives an additional boost in mpg because the larger diameter tires have a lower rolling resistance coefficient. This effect is smaller, but still important.

Ernie Rogers
 

IXLR8

Veteran Member
Joined
Mar 9, 2003
Location
Cushing, ME
TDI
12 Passat Platinum Gray, 02 Golf Black, 01 Jetta Black
It was my understanding that with a Vag-Com you could recalibrate your odo/spedo to get either one of them correct.. but not both.
 
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