I don't know if it will help fuel economy or not. I just bought 235/55/18 Goodyear Assurance Comfortred tires for my Mustang. Stock tire size is 235/50/18. The tire is slightly taller than the stock tire. What it has done is throw the computer's mileage and speedometer off. One of the reasons I'm thinking of getting a aftermarket tune. I realized this after cruising Skyline Drive in Virginia and there were mile markers every mile. My odometer was falling behind almost 1/10th of a mile every mile marker. Then I researched tire calculator sites. They tell you the actual speed you are running vs what your odometer says for different tire sizes. Same thing with the mileage. With my tire size difference these sites say when the speedometer says 60 I'm actually going 62.04 and I'm actually traveling about 5% more than the odometer is showing. Plus my new tires are turning 24 less rotations per mile. The good thing about that is the less rotations per mile, the longer the tire lasts.

http://www.discounttire.com/dtcs/infoTireMath.dos

 

Well which is it???

1/10th of a mile is 10% off in my math book...???

 

My odometer was displaying to me it was about 10% behind. The website I visited on Tire Calculators said with my new tire size my odometer is about 5% behind. I am waiting to buy a tune from Granatelli Motor Sports to match my cold air intake and it should adjust my odometer for my new tire size. It's not available yet they say until maybe the beginning of next year. Until then, I need to go about 20 miles along the mile markers to get a more accurate read on it.

 

I don't think it's as simple as this.... if merely putting big tires on would improve MPG then we'd all be driving monster trucks. The Prius has a 'touring edition' which has slightly larger wheels... and it gets worse fuel economy. (The wheel size is really more about giving the car a nicer ride... which it does do.)

You can think of the tire as a kind of 'gear' that makes contact with the road. When you change tire sizes you're effectively changing the gear ratio. That means you could swap out the differential and get the same effect. Except that when you change a gear ratio you're not just changing the top-end speed that could be produced by a given RPM, you're also changing the torque. This means your making it a little harder for the engine to get the car moving and the eCVT has to adjust to higher revs to do the same amount of acceleration.

And as long as we're picking nits.... with all things being equal, (same tire material... just a bigger size), a bigger tire is also a heavier tire *and* has a slightly larger "footprint" of friction on the road. All I'm really trying to point out here is that a change in one area has consequences elsewhere -- whether the consequences exactly cancel each other out or if one change wins over the other I'm not sure.

Could using a larger tire improve fuel economy? The answer could be "yes", but only if the original tire size wasn't optimal. But that means there's _also_ a chance that putting a smaller tire size on the car could improve fuel economy too.

 

Good answer - that is why I suggested droping weight when you go up in tire circumference, to minimize the torque requirement effect of the "higher ratio".

Of course if you just want better MPG, drop the weight (even with the standard tires.) Leave the spare at home, eliminate rear seat floor mats, clean out the trunk, go on a diet, etc. Every 10 pounds helps.

 

I think you guys are missing the phenomenon that goes on here.

A LARGER DIAMETER, SAME WEIGHT TIRE, DOES NOT CHANGE YOUR ACTUAL MPG!
( If it is heavier, it will decrease MPG. )

Also, adding 10 pounds of weight to a tire is WORSE than adding 10 pounds to the inside of the car. There are forces on a rotating body ( tire ) that are not put on a static body ( a box of food on the seat ).

Throw the RPM's and Revs per Mile out the window and think of conservation of energy.
To answer the original question:

Tire size will not have an effect on MPG, everything else equal.
Tire size will have an effect on the Speedometer, and Odometer, and computer estimated MPG.

P.S. Within normal, reasonable limits, tire pressure does not change tire size, just the tire shape. The perimeter or distance around the tire tread per rev does not change with pressure. Only the size of the flat contact patch with the ground changes. Tires ( within reason ( up to 60 psi ) do not "stretch" more than 1mm per 2000mm circumference.
Yes, this was published by the manufacturer, and yes I tested it with a tailor's tape measure around the tire and you can do the same if you wish.

 

Isn't it possible, that with a larger tire (same weight) you will need more torque to turn the driveshaft... the force is applied to the road to move you out at the Edge of a tire... that is a torque force. The distance from the center scales the amount of force required. That would mean the Engine might actually have to work harder to push the car forward....

So you'd end up going faster, but requiring more power. Conservation of energy applies but engines are more efficient at certain RPM/torque combinations. So.... I would THINK you could either help your MPG or hurt it. My GUT feeling is the engineers set the "gearing" system in the CVT planning on P235 70R16 being the most efficient and you'd end up REDUCING your MPG.

 

I agree Tim, after all Ford has gone to the trouble of putting a tiny spoiler on the rear tires on my '09 FEH to improve mileage. I suggest the older FEH/MMH stay with the engineered size tire. The new Michelin Latitudes on the '09 have improved my mileage on the '05 FEH. Same size and lighter weight together with LRR is a winner for better FE.

GaryG

 

As long as the tire size is real close, I don't think it's any difference. Yes, a heavier tire takes more energy to turn but, on the flip side, I think after getting up to cruising speed, the momentum of a bigger tire should take slightly less RPMs to turn it. The tires I put on my Mustang are Goodyear Assurance Comfortred tires sized 235/55/18 and the original tires were B.F. Goodrich 235/50/18. Revolutions per mile for the original smaller tire are 740, the new larger tire is 715. The original smaller tire weighs 30 lb.s, the larger tire weighs 27 lbs. How about that. The new bigger tire weighs less than the older tire. My source for finding that is tirerack.com. But, I bought the comfortreds for a great ride and low road/tire noise and they have bad rolling resistance. I did lose about 2 mpg on long trips on the hwy with the new tires but I got what I wanted...a good quiet ride. I did pump the mileage back up after my next oil change when I put Motor Silk in the engine. So, I'm right back up to where I was before I bought the new tires.

 

I'd just like to point out in this thread that the 225/75-16 tire size hasn't been mentioned. If you look on a large tire site like Tire Rack, there are a lot of choices in this size, some of which should be very LRR because of their tread design and high load/psi rating (D, E, etc.). Plus, the slightly narrower contact patch would reduce resistance. If you had a passenger 225/75-16 tire, it should also weigh slightly less than the stock tire.
I'll be putting some all-terrains on in a month or so in this LT size (I'll need more than 4 ply passenger tires when driving on sharp stuff loaded) and reporting back when the topic comes up again.