Low Rolling Resistance Tires...Which is Best?

Ernie Rogers

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Location
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Hi, guys,

Okay, so I got pretty philosophical last time. This time, I will try to be more practical. Using the "New" Jetta for a model, I tried calculating the changes in mileage you might get by making some changes with the tires. Here are the results:

Case A: Normal Jetta with Mich. Energy tires, Crr = 0.009
--> 47.5 mpg at 60 mph. (38.2 mpg at 70 mph.)

Case B: Bigger tires, change diameter from 24" to 25"
--> 48.1 mpg at 60 mph.

Case C: Raise pressure from "normal" 30 psi up to 40 psi.
--> 51.7 mpg at 60 mph.

Case B+C: Increase tire diameter and raise pressure.
--> 52.2 mpg at 60 mph.

Case D: Buy cheap tires, more resistance, Crr = 0.012.
--> 42.9 mpg at 60 mph.

So, you can see how your choices affect mileage. I bought some cheap tires last fall, returned them for the satisfaction-guaranteed refund a month later, then paid more for the best I could get. I feel better, but I am still paying for the tires.

This forum has an earlier post on how to measure your own rolling resistance-- a good way to settle any questions. My Michelin Energy tires (25.6 inch dia.) at 40 psi were found to have Crr = 0.0065.

Ernie Rogers
 

Ernie Rogers

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Location
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Beetle, 2003, silver
I'm recommening this post for reading by Darthsurplus.
/Ernie Rogers

More philosophical thoughts about rolling resistance--

Let's assume that it is true that the rolling resistance is proportional to the amount of flexing going on in a tire. One measure of the amount of flexing would be the angle formed between the flat contact patch and the tangent to the tire tread face just outside the patch. Let's call this angle "alpha." (If you want to draw a picture, make a circle, then draw a flat spot on it for the contact patch. Alpha is the angle between the original circle and the flat (chord) line, or it's half the angle you get by drawing lines from the ends of the patch to the center of the circle.)

So, then it should be true (our assumption) that the rolling resistance is proportional to alpha:

RR = F alpha,

where F is the "flexing constant" of the tire.

As mentioned before, the load supported by the tire is equal to the tire pressure times the area of the contact patch:

L = P A

This time, we need to further describe the contact patch-- the area of the patch is equal to its length times the tread width, well roughly so:

A = W C

Where W is the width of the contact patch and C is the length, the chord across the circle. Now, I want to invoke a little geometry. The formula for the length of the contact patch is:

C = 2R sin(alpha), or C = 2R alpha

for small angles, that is for a small contact patch. Putting all this stuff together gives a relationship between the load L and the tire pressure, tread width, tire radius R, and alpha:

L = P W 2R alpha

Way up at the top we had a formula for the rolling resistance and now we have a formula for the load. Dividing one by the other gives a formula for the rolling resistance coefficient:

Crr = RR /L

Substituting,

Crr = F /(P W 2R)

Alpha was in both equations and cancelled out, that's cool. This is an interesting formula, but we have to be very mindful of F, the flexing constant. This "constant" depends on the construction of the tire. There is experimental evidence that it Might be true that F is proportional to the tire width, which is just about the same as W. This means that a tire's rolling resistance coefficient Might not depend on how wide the tire is. (But, remember, the aero drag DOES depend on tire width.) The only other new variable to talk about is the tire radius, R, or the tire diameter, 2R. Now, we have an expanded very interesting relationship for rolling resistance and tire properties:

Rolling resistance is proportional to one over pressure, and one over tire diameter.

Provided that we are talking about tires of identical construction except for the tire diameter.

Nothing has been said here about "dynamics", how the tire and vehicle behave when the car is accelerating, turning, and hitting bumps. (We have lots of those in Pleasant Grove, Utah.)

When choosing tires, dynamics considerations can affect choices too.

Ernie Rogers
 

Ernie Rogers

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About the cases A through D in a post above--

Darthsurplus reminded me that I overlooked a detail. The results I calculated only included tire properties. The effect of larger tires is slightly greater that calculated because the engine efficiency goes up also.

Ernie Rogers
 

DarthSurplus

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Joined
Mar 19, 2004
Location
Indiana
I wonder a little about the L = PA assumption. It seems a little too tidy to be real.


Something you all need to consider before you air your tires to the max and beyond:

Even though the formula for the tangential (horizontal) friction force vector does not include surface area, the realities of life do. Road and tire anomolies reduce the coefficient of friction in various regions of the contact patch. Lowering your tire pressure, and thereby enlarging your contact patch, provides you with more chances to find regions of good traction when pushing the limits. People have slidden off the road in wet but otherwise manageable conditions because their tires were inflated to the maximum number on the sidewall, but the vehicle was not fully loaded. Please consider the risk of becoming a slide-off when you choose to fully inflate your tires.
 

vwxyzero

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” Case B: Bigger tires, change diameter from 24" to 25"
--> 48.1 mpg at 60 mph.”


And over-sizing your tires does what to your odometer reading?
A false idea of your actual MPG achieved?
 

DarthSurplus

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Location
Indiana
And over-sizing your tires does what to your odometer reading?
A false idea of your actual MPG achieved?
Come on. You aren't paying attention.


Ernie posts all of these ideas, calculations and data, the samples of which, that I've observed, do nothing to detract from his claim of being a physicist who experiments with fuel economy. He writes like a nerdy engineer
and he plans experiments like a scientist.

There's nothing there to indicate that he can't or won't or won't remember to compensate for the larger tires in his odometer readings.
 

vwxyzero

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And over-sizing your tires does what to your odometer reading?
A false idea of your actual MPG achieved?
Come on. You aren't paying attention.


Ernie posts all of these ideas, calculations and data, the samples of which, that I've observed, do nothing to detract from his claim of being a physicist who experiments with fuel economy. He writes like a nerdy engineer
and he plans experiments like a scientist.

There's nothing there to indicate that he can't or won't or won't remember to compensate for the larger tires in his odometer readings.
And your the one that made the comment about being carefull about sliding off the road with tires that when over inflated to the limit essintially become motorcycle tires?
S----------S
-L--------L-
--I------I--
---C---C---
----K-K----


Come on what


My point was that the effect it has on your odometer reading is not being addressed at all
 

Ernie Rogers

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Location
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Okay, Okay, I'll mention it, but VWXY0 will be sorry!

When I made up the examples, I was thinking about actual speeds and MPGs, not what your gages read. And, I only took into account the effect of the tires directly, no adjustment for changes in engine efficiency when changing tire size. (Bigger tires get lower RPM and higher torque, so better engine efficiency, actually very important.)

So, according to the request, here's what happens if you read the gages and don't think about the consequences. (Fixed engine efficiency.) Assume that the mileage for case A is what the gages give you, but the speedometer was reading 5% high, and the odometer was right on, no error--

Case A:
Travel 47.5 miles at "60" (actually 57.1) and use 1.0 gal.
You think you got 47.5 mpg, and you did.

Case B: Bigger Tires
Travel "47.5" (actually 49.5) mi. at "60" (actually 59.5). The actual car speed increased by 2.4 mpg, causing increased aero drag. At the same time, the rolling friction dropped by a factor of 24/25 = 0.96 for the tires. The resulting fuel use is 1.09 gal. Dividing, 47.5 /1.09 = "43.5 mpg" about. That's what you think you got. The actual mileage was 49.5 /1.09 = 45.4 mpg. At any rate, the car seems much worse with the new tires. The careless gage reading sunk us.

In reality, if the two tests had been done at the same true speed, the Case B mileage would have been about 0.6 mpg better with the bigger tires simply because they have less rolling resistance.

Sorry, it's probably about as clear as mud.

Ernie Rogers

I'm sure he as taken the odometer issue into account as a matter of course, and not found it worth mentioning.
 

vwxyzero

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Thank you Ernie!

Sorry I asked, not at all.
Muddy, not even.

I did some calculations of my own based on the circumference of the tires mentioned: Case A and Case B and came up with similar numbers, but from a different angle.
24” tires: 840,338 revolutions per tire for every 1000 miles driven.
25” tires: 806,745 revolutions per tire for every 1000 miles driven.

Where I was off and concerned was in not knowing the discrepancies in the speedometer/odometer equation.
Thanks for clearing that up, and making it an aspect of this topic.

I would not have posted in this forum if I wasn’t interested in the topic. For the record I run Biodiesel and am interested in economy issues/topics too.

Also for the record, I think overly inflated tires are a hazard to the driver as well as others on the road if the tires in question don’t hold the road.

And, as a final note, I am very curious how oversized a tire anyone can run before it puts to much strain on the drive train, clutch, etc. etc.

Thanks again for your clarification, and your efforts, vwxyzero
 

RIP TDI

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There are other consequences of fitting larger OD tires besides obvious clearance issues. To name just two: assuming the same wheel offset and alignment specs., a larger tire OD will increase negative scrub radius (distance from SAI road intersection point to the tire's contact patch center, when viewed from the front or rear) and increase caster trail distance (from SAI road intersection point to tire contact patch's center, when viewed from the side). Increased negative scrub radius is desireable from a safety perspective, less desireable from a performance perspective. Increased trail distance is generally desireable from both.

As to higher tire pressures, another consideration besides a narrowed contact patch is that some tire tread construction will not tolerate significantly higher pressures in terms of even treadwear. OEM TDI tires generally perform well in this respect, but not all tires do. I had a dramatic example of this; I had set of Firestone Firehawks in a correct +1 size for one of my cars. Running 40 PSI (rated @ 44 max) for 5000 miles wore the center of the tread down to the wear bars while the outer edges were not even down a 1/32" from new on all 4 tires (car had been correctly aligned right before tire installation). These tires were quite highly rated for real world treadwear according to Tirerack's owner survey (and UTOG was 500). I think this case may be unusual, but I've read reports of less dramatic uneven treadwear with other tires.
 

DarthSurplus

Well-known member
Joined
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Location
Indiana
Chris,

Good point about the uneven tread wear. Changing tire size from OEM also can affect handling, in the form of tendency to drift (the road camber induces the wheels to turn) and in the sensitivity of the steering wheel.


Ernie,

Assume that the mileage for case A is what the gages give you, but the speedometer was reading 5% high, and the odometer was right on, no error
This confuses me. I have no knowledge of Volkswagens. Is this possible on them? Is it possible to alter a computer parameter to adjust the odometer for a different tire diameter, but the speedo to still be off? This implies a separate variable parameter for the speedometer, or a fixed constant. How are VW's computers set up in this regard? I simply don't know. Ford and Chevy correct both the odometer and the speedometer with one parameter. I'm aware of another vehicle, exactly what escapes me right now, that has a correctable speedometer, but the odometer can't be adjusted.
 

RIP TDI

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Not to speak for Ernie, but I assume he was making a hypothetical case for the purpose of illustrating his point.

You're right...the VSS signal is the only driver for both speedo and odo, and their response to the signal cannot be split electroncially without re-engineering the cluster, but it can be split mechanically for a certain type of error. I have corrected the typical VW 3-8% speedo optimism by reindexing the speedo needle on its spindle. As it happens I have also experimented with larger OD tires and installed a pulse frequency adaptor inline with the VSS signal to correct the odo readings as well as my (previously corrected) speedo readings.
 

Ernie Rogers

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Location
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WOW! I'm really impressed by the contributors on this topic. The case I chose was simple, but close to my perception of reality. Checking notes, my car with factory tires was reading 1.1% low on the odometer and 3.4% high on the speedometer. After changing tires to the larger size, the speedometer appears to be right on and the odometer is reading 4.6% low.

So far, my Michelin Energy MXV4 S8 tires appear to be wearing evenly, more variation from tire to tire. I'm depending on the service guy for this info. My tire pressure is about 40 psi cold.

We didn't talk much about the effect of tire size on engine efficiency. According to my engine program, based on the engine map and polynomial fit posted here somewhere, here is the efficiency effect, assume about 60 mph:

24 in. tires, assume 2100 RPM: Eff = 0.332 at 3.4 kW /cyl
25 in. tires, now at 2016 RPM: Eff = 0.339 at 3.4 kW /cyl

That translates to about +1 mpg for me.

By the way, the power per cylinder translates to 18 hp from the engine, a pretty close guess to power required at 60 miles per hour (true speed-- man, this gets complicated).

Ernie Rogers
 

SUNRG

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to return briefly to the thread topic ...

I received this response from Nokian regarding the Nokian WR:

my email: <ul type="square"> On the www.nokian.com website it states:

Nokian WR is the world’s fastest friction tyre…

What does this statement mean? Low rolling resistance?
[/list] their response: <ul type="square"> Dear Rob

That statement was made when we introduced W-rated (max.speed 270 km/h) Nokian WR and that time it was the fastest friction tyre in the world. I really recommend to test Nokian WR cause it`s really is enviromental tyre ; made without any harmful high aromatic oils ( first in the world ), have very low rolling resistance and wet grip ( because of silica tread compound).

Best Regards :

Matti Morri
Manager
Technical Customer Service
Nokian Tyres plc
matti.morri@nokiantyres.com
phone +358 3 3407621
mob. +358 40 517 4444 [/list] <ul type="square">


Nokian WR – the world’s fastest friction tyre

Nokian WR is the world’s fastest friction tyre that has been designed especially for the changing winter conditions of Central Europe.

Safety in all weather conditions

Nokian WR has good grip in the rain and also when you get surprised by snow. Nonetheless, its driving comfort is excellent on a dry road as well. The tyre’s groove angle has been tailor-made for each size, providing more accurate driving and effectively preventing aquaplaning.

A tyre for heavy use

Nokian WR’s new tread can withstand a lot of wear. With the advanced HAKA sipes, the tyre’s properties remain excellent throughout its service life.

Quiet driving pleasure

If you believe a friction tyre to be quiet, Nokian WR will gladly confirm your belief. The tyre’s drive-by and cabin noise are quiet and pleasant.[/list] <font color="red">the tread depth is 12.5/32 compared to 9/32 on new Michelin Energies - that's 38% more tread!</font>

the cost is identical to Michelin Hydroedge ($100 ea + free ship at www.discounttiredirect.com) Nokian WR ($99.99 + free ship at www.tirefactory.com) - and i think i'm leaning toward these, since i absolutely love our Nokian winter tires, these seem like they'll roll well, provide great traction / safety, last long, and they're environmentally friendly.

the Nokian NRHi seem good too, but they're not as long wearing - and at 35-40k/year - we <u>need</u> long wearing.
 

Ernie Rogers

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Joined
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Location
Pleasant Grove, Utah
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Beetle, 2003, silver
Okay, you will get Nokian tires-- now, what size?

Would you care to attempt a measurement of some sort, such as mpg before and after, or maybe a coastdown test?

Ernie Rogers

to return briefly to the thread topic ...

I received this response from Nokian regarding the Nokian WR:

my email: <ul type="square"> On the www.nokian.com website it states:

Nokian WR is the world’s fastest friction tyre…

What does this statement mean? Low rolling resistance?
[/list] their response: <ul type="square"> Dear Rob

That statement was made when we introduced W-rated (max.speed 270 km/h) Nokian WR and that time it was the fastest friction tyre in the world. I really recommend to test Nokian WR cause it`s really is enviromental tyre ; made without any harmful high aromatic oils ( first in the world ), have very low rolling resistance and wet grip ( because of silica tread compound).

Best Regards :

Matti Morri
Manager
Technical Customer Service
Nokian Tyres plc
matti.morri@nokiantyres.com
phone +358 3 3407621
mob. +358 40 517 4444 [/list] <ul type="square">


Nokian WR – the world’s fastest friction tyre

Nokian WR is the world’s fastest friction tyre that has been designed especially for the changing winter conditions of Central Europe.

Safety in all weather conditions

Nokian WR has good grip in the rain and also when you get surprised by snow. Nonetheless, its driving comfort is excellent on a dry road as well. The tyre’s groove angle has been tailor-made for each size, providing more accurate driving and effectively preventing aquaplaning.

A tyre for heavy use

Nokian WR’s new tread can withstand a lot of wear. With the advanced HAKA sipes, the tyre’s properties remain excellent throughout its service life.

Quiet driving pleasure

If you believe a friction tyre to be quiet, Nokian WR will gladly confirm your belief. The tyre’s drive-by and cabin noise are quiet and pleasant.[/list] <font color="red">the tread depth is 12.5/32 compared to 9/32 on new Michelin Energies - that's 38% more tread!</font>

the cost is identical to Michelin Hydroedge ($100 ea + free ship at www.discounttiredirect.com) Nokian WR ($99.99 + free ship at www.tirefactory.com) - and i think i'm leaning toward these, since i absolutely love our Nokian winter tires, these seem like they'll roll well, provide great traction / safety, last long, and they're environmentally friendly.

the Nokian NRHi seem good too, but they're not as long wearing - and at 35-40k/year - we <u>need</u> long wearing.
 

SUNRG

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Location
Roanoke, VA
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None currently. Previously owned 04 Golf TDI & 05 Passat GLS Wagon TDI
Okay, you will get Nokian tires-- now, what size?

Would you care to attempt a measurement of some sort, such as mpg before and after, or maybe a coastdown test?
I do keep track of fuel economy and will perform coast down tests. I know you posted a coast down test procedure somewhere - could you post or PM me a link?

Regarding size, I was planning on sticking with stock 195/65/15 width= 201mm, diameter = 635mm, rotations per mile = 831, and weight = 19.2 lbs

if i went to 205/65/15 the corresponding numbers would be: 208, 648, 816 and 20.9

i'd benefit from fewer rotations / larger diameter, but i imaging the increased weight and width (larger contact patch) would hurt economy. what are your thoughts? changing wheels at this time is not an option so 15" is what we're working with.
 

SUNRG

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i just realized i could go with 205/70/15 tires, that would yield a 34mm tire diameter increase (~1.3 inches), dropping me down to 788 revolutions per mile (from 831) which is a 5.2% reduction.

i think when i'm cruising at 60mph the RPMs are ~2000. 205/70/15 would drop that to 1896.

but, the 205-70s are 2.7lbs heavier (21.9 vs 19.2) and 7mm wider.

OK everyone, what say you! would going to 205-70-15 significantly help fuel economy, or could the increased weight cause the reverse effect?
 

mrGutWrench

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i just realized i could go with 205/70/15 tires,(snip)
__. I'd be very careful going a larger tire percentage aspect *and* a larger size at the same time. Having a tire hit a wheel liner or suspension component could spoil your whole day. (My guess is that it would be OK but I'd want to be SURE!
'
 

SUNRG

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__. I'd be very careful going a larger tire percentage aspect *and* a larger size at the same time. Having a tire hit a wheel liner or suspension component could spoil your whole day. (My guess is that it would be OK but I'd want to be SURE!
'
since the diameter increase would be 34mm, the radius would only increase 17mm; and width increase of 7mm means is just an added 3.5mm on each side of the wheel - so i too am optimistic that this would be OK.

FWIW - the width of 205-70 and 205-65 are identical (both are 208mm - a 7mm increase from the 195-65-15 201mm width). going from 65 to 70 (in 205-xx-15) <u>only</u> adds 21mm to the diameter.

i was thinking the 205-65-15 would not be worth it since it only increases the diameter by 13mm over 195-65-15 - but the increase of 34mm that 205-70-15 yields may be enough to see a fuel economy increase.

the Nokian WR would cost the same in both the 196-65-15 and 205-70-15 sizes, and of course both are long wearing & very low rolling resistance, and 21.9 is not heavy (in fact it's quite lite for the 205-70 size).

by comparison, the GY Assurance Triple Tread weighs 24# in the 195-65-15 size and 26# in the 205-70-15, and in those respective sizes the Michelin Hydroedge weighs 21# and 24#.
 

RIP TDI

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OK everyone, what say you! would going to 205-70-15 significantly help fuel economy, or could the increased weight cause the reverse effect?
Depends on driving conditions. The more time spent at steady cruising speed and the less time spent accelerating the greater inertia of the larger effective rotating mass, the greater the chance for fuel efficiency gain. Exclusive highway driving should show some increase, pure city stop & go may show none or a loss.
 

nicklockard

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Me three. If ya cruise you'll see improvements due to the lowered effective final drive ratio, as Ernie has. (Plus you're engine will operate closer to it's BSFC peak "ideal" and thus be more efficient." If you do stop-and-go and lots of city driving...probably a wash, due to acceleration losses counteracting the above benefit.
 

JerryPT

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Okay, larger diameter tires have been discussed here. What about narrower tires to reduce the contact patch? It seems the hybrids have taller, skinnier OEM tires than most.

I'm wondering about going from the stock 195/60/14 on my 96 Passat to 175/65/14. My old Subaru wagon, a much lighter vehicle, had 155's I remember.

Jerry
 

tditom

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Jerry-
I was wondering the same about my 97 Passat, but was only thinking about going down to 185/65R14. This would be nearly the same diameter tire, but would have a bit less contact patch.

This tire has an "approved" rim width of 5-6.5", and a "measuring" rim width of 5.5" (these terms from the Nokian spec sheet). The original 195/60R14 tire is listed at 5.5-7" and 6", respectively. Does anyone happen to know if the 185's will fit the OE 6x14 steel rims?

My other question is regarding VW's spec for the tire size. My tire pressure label only mentions 195/60R14 and 205/50R15. The owners manual says nothing about acceptable tires except that they should match the replaced ones. But my Bentley does list the 185/65R14 as a suitable replacement. Does anyone see any reason why I couldn't use this size tire? Thanks for any input.
-tom
 

JerryPT

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tditom:
KS97 Passat is running the 185/65/14s in a Bridgestone tire. He's posting in the Tires and Fuel Mileage thread, and is happy so far. He took the Bentley at its word. I'm ready to do the same. I believe they will fit the stock steel rims.

Jerry
 

moondawg

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i just realized i could go with 205/70/15 tires, that would yield a 34mm tire diameter increase (~1.3 inches), dropping me down to 788 revolutions per mile (from 831) which is a 5.2% reduction.

i think when i'm cruising at 60mph the RPMs are ~2000. 205/70/15 would drop that to 1896.

but, the 205-70s are 2.7lbs heavier (21.9 vs 19.2) and 7mm wider.

OK everyone, what say you! would going to 205-70-15 significantly help fuel economy, or could the increased weight cause the reverse effect?
I'm about to find out! I stopped at my local tire place and ordered 4 WR's in the 205/70/15 size. They will replace my 195/65/15 Energies.

Should be here soon.

moondawg
 

moondawg

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If my stock speedometer weren't already about 5% fast, that would be a problem. Now it will be right, and my odometer will be wrong.

moondawg
 

DrSmile

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I would not at all be surprised if a 1.5cm taller FULL TREAD new tire would rub. I'd say chances are real good it would.
 
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