I knew there was a website that if you punch in your old size tires and your new size tires it will give you how many MPG you are actually getting. Does anyone have that website.
I had 235/70R/16's and put on 285/60R18's.
Thanks alot.

 

MPG as in miles per gallon? There is no mathematical formula to compare that. If you mean MPH, then try this one. Looks like you are off about 8.7% on the slow side. That means, if your speedo say 60mph, you are actually going 65mph.