size of wheels will affect gas mileage, also affect your speedometer. your mph will not be acurate.
but having bigger wheels, im guessing you want some 22s and up? those wheels just weigh too much, weight is a big factor when it come to mileage. some will tell you bigger wheels will give you better mileage because one rotating of a 22in wheel goes further then a stock 16inch wheel. which would be true if they were the same weight. but they almost never is.
on my cadillac deville, i lose about 20miles per tank on 20s compared to stock 16s.