Better MPG with 15" tires than 17" is this true?

No he said "IF"

 

Yes.

The OEM 17" (215/45R17) are wider and lower profile (tire height 45% of the 215 mm width). The lower profile is enough to make the outside diameter at face of tread less, to the tune of 1~2 % more revolutions per given distance.

In other words, with the 17" setup the wheels are spinning a little faster at any speed.

Some people are replacing the OEM 17" with 205/50R17, which is virtually the same OD as 195/65R17, actually slightly bigger.



(the above's just extrapolating from the tire data. Reality, especially revs per mile, might vary a bit.

 

While I agree in general, I wanted to point out that the official ratings for revs/mile on the above tires are:

215/45/17= 844
205/50/17=828
195/65/15= 829

Each model will be slightly different but most are within 2-3 of the numbers I listed.

Yes, 215/45/17 tires are smaller in outside diameter than the 195/65/15. Don't let the wheel diameter trick you.

 

Roughly correct. There seems to be a bias in the displayed MPG of about 5 - 6%. So, if you are displaying 60 mpg, then you can expect that your "calculated" mpg to be in the 56.5 - 57 mpg range.

For the 70,000 miles I've tracked for my car, my lifetime offset is 5.8%.

 

My error, over 37000~ km's (virtually since new) is 7.77%. The error percent seems to be lower on the better mileage tanks, fwiw.

(our overall 5.1 liters per 100 km equates to around 46 mpg)

I just divided the circumference into a mile, obviously more going on with the variation, likely your numbers based on real tests.

 

Aye, tires deform while rolling and that skews the numbers so you can't do simple math.

 

Muhahaha!!!!, I beg to differ my Prius has 17" wheels and I still achieved this!

And here is my car...

 

^^ He cheats! He has his buddy push him around the block to get those numbers!