I'm in the market for a set of aftermarket rims. My car has 16'* on it. If I go to 18'* will the speedo be off? How do you adjust this?

with 18'* it depends what the diameter of the wheel and tire combination to see just how off it will be if it'* the same diameter as the 16'* with the stock tires then you should be all set. and for computer controlled speedo'* the only real way to do so is with a computer chip or flash or something along those lines 9 times out of 10 it wont be much of a change if not a mile or two.

Your stock tires are probably 225/60X16 which are 26.63 inches tall. A 235/50X18 is 27.25 inches and a 245/45X18 is 26.68 inches high. Either would work since your speedo is slightly optimistic by about 3 to 5% from the factory. [70 mph indicated is actually 67 to 68 true mph] These larger tires with their short sidewalls will have a very stiff ride and usually the largest tires have low treadwear ratings and may wear out sooner. Hope this helps.