Tires and MPG

VaanBasch

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97 Jetta GL IDI AAZ
So I have this lovely little 97 Jetta IDI that I'd like to squeeze just a couple more MPG out of ... It is standard , straight piped ( cat back) ... Anyway .. Current tires are 185/65r/14 ( May be 70). If I was to change them to 185/ 70r /14 what would this do for mpg ?
 

vanbcguy

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Nothing at all other than make your speedometer / odometer read incorrectly.

Look for lightweight low rolling resistance tires and keep them inflated to their rated pressure, that will give you the best MPG possible from tires alone.
 

PeteZ06

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I disagree! The taller the tire the lower the engine rpm at same speed. I've been thinking of doing the same mod but current tires still have plenty of life in them.
 

VaanBasch

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97 Jetta GL IDI AAZ
I agree with keeping correct psi in the tires , and still unsure about the height ... I have a protege and a probe ... Same engine ( more or less ) and same transmission ... The probe had taller tires. At 60mph the probe does apx 2100rpm and the protege will be at apx 2600 rpm ... I have no fuel mileage comparisons other then Mazda is about 26mpg in the summer ( Canadian gas changes in the winter)
 

donDavide

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I have never gotten better from going from 36-38 to 45 or even max, I tried that after TDI fest in Ohio after the hypermile guy gave his lecture, I got worse mileage on the way Home, and it was downhill too!!!
 

kjclow

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I have never gotten better from going from 36-38 to 45 or even max, I tried that after TDI fest in Ohio after the hypermile guy gave his lecture, I got worse mileage on the way Home, and it was downhill too!!!
I tried that too and didn't see enough difference to matter. The low rolling resistance tires did make a +5% difference on the JSW.
 

Diesl

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I have never gotten better from going from 36-38 to 45 or even max, I tried that after TDI fest in Ohio after the hypermile guy gave his lecture, I got worse mileage on the way Home, and it was downhill too!!!
I was lucky that I did the tire pressure increase right around the annual temperature minimum in February, so it was easy to see the step in MPGs, because there was no seasonal MPG variation to deal with.
 

Concat

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I disagree! The taller the tire the lower the engine rpm at same speed. I've been thinking of doing the same mod but current tires still have plenty of life in them.
Sure, but for a given torque value you are applying less force to the pavement (lower acceleration). The force vector is further away from the axis, and is therefore smaller. Moreover, the moment of inertia is higher. In short, for any given RPM, you need more fuel.

And sure, you'll see lower RPM for a given speed... but you're probably using the same amount of fuel in order to apply the same amount of force to the road and maintain speed or accelerate at the same rate.

If it was a simple as taller tire = better fuel savings, you'd think the auto industry would have caught on. There are many factors to consider here beyond what I just stated.
 
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SilverGhost

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And just to throw a giant monkey wrench in the works I get 8-12MPG swings through out the year, but less than 2 MPG difference going from my 195/65R15 to 225/40R18. Some days are even better with the 18" but most days about the same as the 15".

Jason
 

donDavide

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And just to throw a giant monkey wrench in the works I get 8-12MPG swings through out the year, but less than 2 MPG difference going from my 195/65R15 to 225/40R18. Some days are even better with the 18" but most days about the same as the 15".

Jason
My B5.5 came with 195/65-15, I went to 205/55-16 (goodyears to Kuhmo) and milage improved. So did handling.
 

PeteZ06

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If it was a simple as taller tire = better fuel savings, you'd think the auto industry would have caught on. There are many factors to consider here beyond what I just stated.
It is as simple as that.. Simple question.. Does your car burn more gas cruising at 2300rpm or at 2000rpm? See.. Its that simple.

A taller tire isn't always the answer to everything.. You will have worse initial acceleration.. Worse handling and worse braking. But yes.. At highway speeds you will definitely improve mpg.
 

PeteZ06

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My B5.5 came with 195/65-15, I went to 205/55-16 (goodyears to Kuhmo) and milage improved. So did handling.

Its not only about the wall height but overall diameter of the tire... Since you're going up in rim size youre more than likely increasing the diameter even with a shorter wall.
 

Joester

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Concat, what you're missing is BSFC, or brake specific fuel consumption.

You are correct in saying that your car has to create the torque needed to drive the car at a given speed regardless of tire size, but internal combustion engines are more efficient in converting fuel to power at certain RPMs than others.

So even though the engine has to create the same amount of power to move the car down the road at 60mph, it may do so more efficiently at 2000rpm than at 2500rpm, for example.

Now what is the best tire size/RPM combo so that you minimize both moment of inertia of the tire/wheel combo AND bsfc? That takes a some math and a BSFC chart of your specific engine such as the one shown below. (y-axis is MEP i think)

This specific engine (dont know what it is, just pulled a bsfc chart from google) is most efficient at ~2250rpm and 15bar MEP

 
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nicklockard

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Aug 15, 2004
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So I have this lovely little 97 Jetta IDI that I'd like to squeeze just a couple more MPG out of ... It is standard , straight piped ( cat back) ... Anyway .. Current tires are 185/65r/14 ( May be 70). If I was to change them to 185/ 70r /14 what would this do for mpg ?

If you combine it with running ~44psi front 38psi rear, it will net about 4% better fuel economy.

Remember that you'll have to perform an odometer correction factor since your final drive ratio will change by the factor of (new diameter)/(old diameter)...this will the the error. The corrective factor will then be

Apparent Odometer reading * old dia/new dia = correct number of miles.

Actually the best way to correct for odometer errors is to drive a well known distance by GPS or maps confirmation and compare to the apparent miles traveled by your odometer.

Besides pressure, tire compound makes a HUUUUUUGE difference. Look for low rolling resistance, high silica content tires. They do cost more, but also return better FE.
 

VaanBasch

Active member
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Jul 26, 2014
Location
Ontario
TDI
97 Jetta GL IDI AAZ
I'm usually pretty fussy about tire brand and avoiding tires with a "V" like pattern ( better grip but higher rolling resistance), as well avoiding "B" and lower ratings. This little beast came with some odd Indonesian made tires ... In the end I'd want some tires made in Canada , USA or an Eastern European country and if it saves me a few bucks with them being a 65 vs a 60 for height % .

I think from what I have read off this site and listened to from a few non sales reps in the tire industry. The factors in a MPG gain can be gained easy with simple things that should be a told to everyone ;

tire pressure ~3-5% increase in mpg

Tread type ~1-2% mpg *

Brand ... No numbers, but quality brand generally gives better initial balance

Tire age ! Old tires have been sitting around and decaying and getting harder and cracked ( possible dangers with weather cracking)


Anything to question or add ?

* "V" type traction example nexen s2000 etc.
 

Lightflyer1

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Round Rock, Texas
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Concat, what you're missing is BSFC, or brake specific fuel consumption.

You are correct in saying that your car has to create the torque needed to drive the car at a given speed regardless of tire size, but internal combustion engines are more efficient in converting fuel to power at certain RPMs than others.

So even though the engine has to create the same amount of power to move the car down the road at 60mph, it may do so more efficiently at 2000rpm than at 2500rpm, for example.

Now what is the best tire size/RPM combo so that you minimize both moment of inertia of the tire/wheel combo AND bsfc? That takes a some math and a BSFC chart of your specific engine such as the one shown below. (y-axis is MEP i think)

This specific engine (dont know what it is, just pulled a bsfc chart from google) is most efficient at ~2250rpm and 15bar MEP

IIRC those charts are at 100% load as well. Partial loads may be different.
 

Joester

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IIRC those charts are at 100% load as well. Partial loads may be different.

x-axis is speed
y-axis is load (MEP, which scales with torque)
z-axis is fuel consumption

Heres another one thats a bit easier to read. That top line, of course, is 100% load... the max amount of torque the engine can produce at that given RPM.

 
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Oilerlord

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For what it's worth...Tire rack did a comparision of popular low-rolling resistance tires. Bridgestone Ecopias looked like the highest MPG tire at the time.

http://www.tirerack.com/tires/tests/testDisplay.jsp?ttid=155&affiliate=HK8

If you look at the results - the difference in MPG is at best...subtle. For me, I was looking for a tire with decent FE but low road noise and improved handling / braking performance was more important. I went with a set of Michelin Pilot Sport AS/3's. Amazing tires.
 

Concat

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Location
Edmonton, AB
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2005 Jetta GLS TDi
It is as simple as that.. Simple question.. Does your car burn more gas cruising at 2300rpm or at 2000rpm? See.. Its that simple.

A taller tire isn't always the answer to everything.. You will have worse initial acceleration.. Worse handling and worse braking. But yes.. At highway speeds you will definitely improve mpg.
No, it's really not. If all we are talking about is highway speeds, then sure... but there's still a limit (see Joester's post about BSFC). If your tire is so big that your RPMs somehow get down to 1500 at highway speed, then your fuel efficiency is most likely worse. That and your FE in the city would be worse too.

You have to balance all factors... and there are a lot. I'm not saying a taller tire WON'T improve mileage... I'm saying that it won't in all circumstances, and that you can't just keep going taller and taller and expect better and better results.

You're oversimplifying things when all you think about is RPM and how many engine strokes per mile... surely we can agree on that.

I wonder how many mpg this guy gets:

 
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donDavide

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I have never gotten better from going from 36-38 to 45 or even max, I tried that after TDI fest in Ohio after the hypermile guy gave his lecture, I got worse mileage on the way Home, and it was downhill too!!!
Plus with the roads In DC and some suburbs, it takes pounding, so I don't want the tires to high.
 

kjclow

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Location
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2010 JSW TDI silver and black. 2017 Ram Ecodiesel dark red with brown and beige interior.
For what it's worth...Tire rack did a comparision of popular low-rolling resistance tires. Bridgestone Ecopias looked like the highest MPG tire at the time.

http://www.tirerack.com/tires/tests/testDisplay.jsp?ttid=155&affiliate=HK8

If you look at the results - the difference in MPG is at best...subtle. For me, I was looking for a tire with decent FE but low road noise and improved handling / braking performance was more important. I went with a set of Michelin Pilot Sport AS/3's. Amazing tires.
I put the Ecopias on the JSW and saw a mileage improvement. Driving through PA, MD, and VA in crappy winter weather in 2013, I did not think the tires were lacking in handling. I put the AS/3s on my wife's Golf and her only comment to date is that they seem a little noisier than the OEM Contis. With the downpours we get around here, she's not complained about wet handling since the change.
 

Joester

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Another oddity to consider is that even if your wheel/tire setup weighs 2 tons, it theoretically takes no more energy to KEEP it rotating on the highway. It only takes more energy to ACCELERATE the wheel up to speed.

Because newtons laws of motion also apply to rotating things, just like Force=Mass*Acceleration, Torque=Moment of Inertia*Angular Acceleration. Since cruising at a constant speed means no angular acceleration, torque must also be 0.

The only forces that your engine needs to overcome AT CRUISING SPEED are
-engine friction (RFMEP+AMEP)
-drivetrain losses in gears and transmission
-friction in bearings
-rolling resistance of tires (depends on tire size I suppose, but not really tire weight)
-air resistance of car

so yes, a much heavier/larger wheel and tire setup will net you way less MPG in the city (I could feel and observe a drastic change when I went from a 195/65/R15 tire to 52lb heavier setup with 225/45/R17, but on the highway it shouldn't have nearly as much of an effect, if any at all. For example, my best mileage I've ever recorded has been 52MPG and that was with 17x7 rims and sticky summer tires, but it was a 100% highway tank where weight of the wheel/tire setup doesnt really matter as much as rolling resistance and cruising speed.
 
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1854sailor

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Tom Brown

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I have never gotten better from going from 36-38 to 45 or even max, I tried that after TDI fest in Ohio after the hypermile guy gave his lecture, I got worse mileage on the way Home, and it was downhill too!!!
I've done the same test with the same result as you, however, I would like to point something out.

With higher tire pressure comes larger relevant wheel diameter. As well as reducing the contact patch, therefore, reducing rolling resistance, you are going further per revolution. That is something your vehicle telemetry is oblivious of.

The point being, if you can go a wee bit further with identical reported fuel economy, you are actually getting a bit better fuel economy.

... but I'm going to stick with VW specified pressure. lol! :D
 

PeteZ06

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No, it's really not. If all we are talking about is highway speeds, then sure... but there's still a limit (see Joester's post about BSFC). If your tire is so big that your RPMs somehow get down to 1500 at highway speed, then your fuel efficiency is most likely worse. That and your FE in the city would be worse too.

You have to balance all factors... and there are a lot. I'm not saying a taller tire WON'T improve mileage... I'm saying that it won't in all circumstances, and that you can't just keep going taller and taller and expect better and better results.

You're oversimplifying things when all you think about is RPM and how many engine strokes per mile... surely we can agree on that.
No one is talking about putting on 40" tires.... youre going into the extreme trying to prove your point by overthinking this.

I recently did this with my Corvette because of traction issues and I do 1/2 mile races and needed to adjust my gearing. I went from a rear tire 315/30/18 to a 315/40/18. I gained 1.5" in tire height and 15mph in 4th gear which is exactly what I needed for my races.. At the same time my rpms dropped at highways speeds and my mileage went up according to the DIC.

You can argue your point all you want but me burning less gas in my Vette because of a taller tire is a proven fact.




 
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SD26

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I've done the taller & narrower tire swap twice. When doing it, the first thing to recognize is that tire sizing is a range. Each manufacturer will have different heights, and, therefore, actual circumferences, for their tires. They aren't always the same, and they aren't even necessarily the same for a brand within their line. So, in making calculations for MPG, one would have to account for that first.

Next, I never could seem to find that going taller and narrower helped my MPG. One, making the tire taller will make the vehicle itself taller. Next, more air will flow under the vehicle. I think that resulted in more drag and negated the opportunities of the "improved" gearing. Perhaps, installing an extended air dam would have helped my MPG or, maybe better, lowering the car as that would result in restoring the lower ground clearance in maintaining the frontal area.

I've got a van that is about due for tires, and I'm thinking going narrower and maybe shorter than the current tire height.

YMMV :D
 
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