If you get new tires, you can stagger the sidewall heights of the front and rear to make the total outside diameter of the tires the same as factory. This will solve your speedo calibration problem, and it might look cool. If you look at the specs for the factory tires at Tire Rack, you can find the overall diameter. Try to find something with an 18" rim size and the same outside diameter. I don't know if they exist, but its a place to start.