Hi guys, my fronts are 245/45/18 and rear were 255/45/18... I jus had my rear replaced today with 255/40/18, so basically the rear now sits just a little lower than it did before.

So my question is since now that my rear have a smaller diameter, will it increase or decrease the mpg and how will it affect the acceleration?

 

Acceleration should be a bit better because you've essentially lowered the gear ratios. Gas mileage shouldn't change much, if at all.

That said, you've also got a shorter tire in the rear than in the front - you may get some VDC issues especially if you drive enthusiastically.