Actually as long as he keeps the over all diameter of the tires the same, he won't need to change his speedo. For example, if he has 255/45/18 tires on it now he can use 255/35/20 tires on it and the speedo will remain correct as the over all diameter stays the same. As far as weight goes, it really is going to depend on the wheels that you choose. We sell some wheels, the Godspeed Omega, that weighs less in 20" than the factory 17s and 18s that come on the car(the Godspeeds are around 24lbs each). In many instances if you choose a lighter, larger wheel you not only get a handling increase, you also shed some un-sprung weight weight which can increase your efficiency.
Thanx John!!! You guys have some great prices and selection!!!