[mickmate]

Your speedometer is actually a voltmeter. It simply reads the voltage generated from the small generator inside it that the cable is driving. There is a calibration screw on the back (it might be touched with a dob of paint). You have the right idea already, if you can get an accurate difference at highway speed, lets say your number was 90 and supposed to read 70. You can remove the speedo and put it on a drive like a variable speed drill and bring it up to 90. You can use a universal speedo cable or the one from the car to drive it. Now turn the adjustment calibration screw on the back to make it read 70 and you will be pretty close. The other more expensive option is to have it calibrated on a machine like a dyno or rolling road machine that effectively does the same thing only by spinning the wheels through the drivetrain to get the speed and giving the same screw a tweak.