Speedometer Calibration

[H2-T2]

I have dealer installed, 18 inch wheels and tires. I have noticed that the tires, (Nitto) are 32.99 in. diameter. The stock Hummer tires are 34.5. On a recent trip that is exactly 684 miles, my odometer registered 720 miles. My question is, how is the speedometer recalibrated? Is it a simple matter of the dealer getting into the computer and changing the settings? It is also throwing my speedometer off by about 4 miles at 70 miles per hour. Thanks for any advice. Gary.