I recently changed tires on my Tricross from the stock Specialized 32mm to 28mm Continental Ultra Gatorskins. This is what I did to adjust the speedometer calibration to allow for a smaller tire size.You can do a roll-out measurement where you measure the distance of one revolution, but this is more accurate.

This method works for any speed sensor that is calibrated with a wheel measurement. I'm using a Garmin GSC 10 speed/cadence sensor displayed on a Garmin F50 watch. This computer (as well as the others I have used) uses wheel diameter in millimeters. (Note: It's a Garmin but it does not have a GPS and uses a magnetic pickup on the wheel to determine speed and distance.)

First step, I went for a twenty mile ride. The longer the ride the more accurate the distance measurement will be, although I don't know that it matters after a point. The computer said I went 20.36 miles.

Second step, I put my ride into MapMyRide. I turned off the "Follow Roads" option and turned on the satellite view, to make the tracking more accurate. It gave my distance as I don't know how accurate this is, but I'm confident the measurement is more accurate than I need.

Third step, do the math. My current setting is 2172. That means the computer thinks the bike moves 2172mm forward with every wheel revolution. I traveled 19.8 miles, but the computer thinks I traveled 20.36 miles.

19.8/20.36 = .972 ( = 97.2%)

The computer thinks I traveled farther than I did, so that means that it thinks the wheel is bigger than it is, and the actual wheel size is 97.2% of the current wheel size setting.

2172 x .972 = 2112

The wheel size needs to change from 2172 to 2112. As a check, make sure that makes sense. The computer thought I traveled more than I did because it thought the wheel was bigger than it really is. The new setting is telling the computer the wheel is smaller, so that is correct. If the new number is going the wrong way, either bigger or smaller, it's probably from doing indicated/actual instead of actual/indicated on the division.

Fourth step, take another ride and see how it works.