Tires: Changing tire size and gas consumption., Tire, Fuel Economy

Question
I recently changed tires on my ML 350 from 255/60 R 17 to 245/65 R17.  My gas mileage went from 18 mpg to 13.8 with the first fill-up after new tires, and then to 11.67 mpg after the second fill-up.  Could the decrease in mpg stem from the change in tire size?  I just had it in the shop for an oil change and brakes (after pumping the second tank of gas into the vehicle).

Answer
Sylvania,

Compared to the differences between tires - meaning make and model - simple tire size changes have very little affect on performance.  This includes fuel consumption.

The difference in diameter between a 255/60R17 and a 245/65R17 is about a percent and a half - in the wrong direction, so that may account for some of it.  Put another way, your odometer is now reading low and you are actually going further than you think, so your true fuel economy is better than you've calculated.

Some of it is no doubt that you replaced worn tires with brand new ones.  Just for reference, all other things being equal, worn tires consume less fuel than new, fully treaded, tires.  So every time you put new tires on, you should anticipate some loss of fuel economy.

And lastly, there can be a HUGE difference in fuel consumption between tires (different make and model).  Original Equipment (OE) tires - the tires that come on vehicles from the assembly plants - typically were designed with emphasis on fuel economy.  This typically comes at the expense of traction and/or treadwear.

So many folks replace their OE tires with tires with better treadwear ratings only to find their fuel economy has suffered - and that's the way the physics works.  I'll bet that applies here as well.

Also many folks are not aware of the effect inflation pressure has on fuel economy.  If I were you, I'd be sure to check my inflation pressures regularly:  "They" say once a month!  - and I would do it myself, just to be sure it is done correctly.  The guys at the shop aren't really going to pay attention to you when you are only 1 out of 100 folks they see in a day - and after a while, they get sloppy!

But I don't want you to think that there might not be another cause - one coincidental to the the tire change.  That's possible, so be sensitive to driveablility problems.