Depends on how big an increase. Highway may pick up a bit and city may dip. If the size change is not that substantial you will hardly notice it. You can get a feel for how drastic the change might be by hitting up one of the sites that has a tire calculator and will give you some idea of the differences. The Tire Rack can also get you all the sizes of tires to see how much over stock it is. The reason is a bigger tire makes it like a lower numeric gear ratio. So a bigger tire will cause your engine to rev lower then a smaller one. So highway can really benefit from lower revs, but around town you motor has to work a bit harder now to move the car. Think if you had to start your car in 2nd gear instead of first, similar but not as bad.