[nikoloz]

Hi, MPG will never be affected as long as you will use stock size tires, As you say you are now using 18" wheels and probably 255/55/18" tires (stock) size, now if you are going to use 20" wheels and stick to stock tire sizes you need to buy 275/40/20" and main thing is that overall diameter is the same for 255/55/18" and 275/40/20" Now in most cases guys are fitting 275/45/20" to get a bit comfortable ride but as OD is increasing you get slightly less mpg, Im running 285/50/20" and i can say that its as comfortable as 255/55/18 but i have to sacrifice some fuel for this awesome aggressive look

 

[nikoloz]

Oh and forgot to say that one of the main things you need to take to consideration is the rolling resistance of the tire, As its one of the key factors for getting better MPG kind regards, Nikoloz