Gas milage and Tire size

[TR3-2001SE]

I recently change my tire size to a lower profile and got flatter handling and better acceleration. Is it possible to go the other way?
Is there some sort of breaking point where larger may not give you better milage? Probably when the tires get heavier or wider the milage would start to be affected. Your expert thoughts?

 

[dB]

There's lots to consider. By increasing the diameter, you affect the final gearing of the transmission. This allows you to travel farther given the same number of engine revolutions. Of course, you throw the odometer/speedometer off. You could also run into rubbing issues with the fenders. If you stick with the same diameter, but go plus 1 or plus 2 tire sizing, then you'll get into stickier rubber, which will increase rolling resistance some and reduce mpg. Of course, tire and wheel weight will effect mpg in some small way.

 

[eljefino]

You likely have an overdrive ratio that's about perfect for economy with the standard tire anyway (mild lugging near torque peak at 65 mph with a gasoline engine.) Go taller and you might experience TCC unlocking or even downshifting when faced with the most minor hill or headwind. This hypothesis applies to bread & butter economy cars; some gains might happen with an unloaded truck for example.

 

[CapriRacer]

There are too many variables here!

When you say you went lower aspect ratio, do you mean you went the same rim diameter and overall diameter, but went wider?

Or do you mean you kept the same overall diameter and went to a larger rim diameter?

 

[seotaji]

Random question:

My car orignally had 195/60/15's.

I'm currently using 205/60/15's.

The handling is quite a bit softer, but should I expect a loss of economy (MPG) since the tire is a bit fatter?

 

[JohnnyO]

quote:

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Originally posted by seotaji:
The handling is quite a bit softer, but should I expect a loss of economy (MPG) since the tire is a bit fatter?

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Maybe, but I don't think that's enough to make a difference.

 

[tadaima]

I had 185/60/14 treadwear 440 all season Republic tires on my del sol and went to 195/60/15 Falken Azenis treadwear rating of like 200 and sticky.

I got at best 37mpg with the 185/60/14 and 35mpg with the 195/60/14, I could feel a significant difference between the 2 tires, however they are different class tires, one being cheap all-season and the other being summer-performance.

 

[labman]

Other things being equal, a wider tire should have higher rolling resistance. Pressure and sidewall stiffness may have a bigger effect. You must correct for any changes in overall diameter. A 185/60/14 calculates to 23.74 inches. Not all tires marked as that will have that diameter when new. Putting a tire on that is 3% larger will reduce the speed and mileage reported by the speedometer by 3%. That may not sound much, but in this 9 is fine and 10 you are mine world, 66 mph instead of 64 mph could cost the price of several tanks of gas.

 

[XS650]

quote:

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Originally posted by tadaima:
I had 185/60/14 treadwear 440 all season Republic tires on my del sol and went to 195/60/15 Falken Azenis treadwear rating of like 200 and sticky.

I got at best 37mpg with the 185/60/14 and 35mpg with the 195/60/14, I could feel a significant difference between the 2 tires, however they are different class tires, one being cheap all-season and the other being summer-performance.

---

If the tires were the nominal by the numbers size, than you went 2% further for the same odometer reading. That alone would account for 1 mpg of your 2 mpg change.

And, like labman said, if you drove the same speedometer speed, you were actually going faster at the same speedometer reading, so that used more gas. A 2% increase in speed requires a 6% increase in power to overcome aerodynamic drag.

 

[moribundman]

Increased air resistence due to wider tires is also a factor. When I went from 185 to 195 mm wide tires on my Scirocco, top speed dropped about 5 km/h.

On a compact/medium size car, I consider a 205 mm wide tire the best compromise between improved handling and decreased speed and gas mileage.

Tread pattern makes a big difference when it comes to rolling resistance. Some tires have huge grooves, while others have barely any.

 

[XS650]

quote:

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Originally posted by moribundman:
Increased air resistence due to wider tires is also a factor. When I went from 185 to 195 mm wide tires on my Scirocco, top speed dropped about 5 km/h.

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If you were going by the speedometer and saw a 5 km/h drop with that change with the same profile, the top speed was virtually the same.

 

[moribundman]

Top speed dropped from 200 to 193 km/h per speedometer. I really don't recall by how many percent the speedometer was fast, but I vaguely remember the industry standard was +5% with a +/-2% error margin. More to the point: With the wider tires I could not outrun my friends' Manta and Capri.

 

[JeepZJ4.0]

Ah.. the beauty of an undersized speedo..... the Speedo on the Jeep Grand Cherokee optimized at 28.5. The tires that come with it are 28.2 inches. The tires I have on it now are about 28.7. So it's a little closer.

 

[TR3-2001SE]

OK here is the deal. My original tires where 205/45/16. My milage was near 31 mpg. I changed to 205/40/16. I adjusted the calculation for milage for the undersized tire and actual milage was about 30. I have now installed 195/50/16. I love the taller gearing. My speedometer which was always off is now right on the money. I will report later on milage but it looks to me much improved by the smaller width and larger dia. Before 55mph actual was 2900 rpm. Now 55mph actual and indicated is 2600 rpm. The new tires are also the same weight as the ones I replaced 18lbs.

 

[TR3-2001SE]

Well the milage is 32.5 so it has improved 7% and this is with brand new tires. Should improve as the tires wear in more. I have also switched from high test to midgrade so not sure if that has made any difference. Before I got better milage with the high test shell v power.

 

[TR3-2001SE]

One more thing. The 30mpg was with mobil 1 10w30. The 32.5 was with Mobil 1 0w-40. So increase in tire size equals more mpg. As long as weight is not increased and tire width is not increased.

 

[GC4lunch]

quote:

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Originally posted by TR3-2001SE:
OK here is the deal. My original tires where 205/45/16. My milage was near 31 mpg. I changed to 205/40/16. I adjusted the calculation for milage for the undersized tire and actual milage was about 30. I have now installed 195/50/16. I love the taller gearing. My speedometer which was always off is now right on the money. I will report later on milage but it looks to me much improved by the smaller width and larger dia. Before 55mph actual was 2900 rpm. Now 55mph actual and indicated is 2600 rpm. The new tires are also the same weight as the ones I replaced 18lbs.

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But how did you "adjust the mileage for the undersized tire," and how do you know that your speedometer is "now right on the money"?

If you used any of the many "tire size calculators" in the Web, they all have a precision of +/- five percent or worse. And none (that I have found anyway) takes into account that a tire mounted on a rim that has a different width than the "measuring rim" specified by the manufacturer has different dimensions than the dimensions stated on the sidewall.

 

[TR3-2001SE]

I did use the tire size calculators on the web for the undersized tire. I don't think the rim width unless extreme would make any difference in overall dia of the tire. The overall dia according to tirerack was identical to the tire size calculator also. Now I have adjusted the milage based on the measured mile signs that are on the side of the road. The new tires have the odomoter right on the money over 20+ miles. Trip milage has gone up to 36mpg driving 65 mph. This on a car rated by the EPA of 28 highway. This was also on regular gas where midgrade is recommended. Speedometer is now right on the money based on highway department road side speed indicator and gps. Don't know how it could be more accurate?

 

[DriveHard]

might want to find a friend with a GPS to confirm your calculations.