Changing from 17" to 15"

[Neurotic Hapi Snak]

A larger diameter means less revolutions per a given distance (effectively a higher gear ratio) but you run into fitment issues with larger diameters or they rub when turning or going over bumps. It's a good idea to stick close to the stock tire diameter. Also with offset, the lower the offset, the farther the tire "sticks out", too low and you will rub your fenders on bumps. Too high and the wheel might hit the suspension.