You should calibrate it to the speed you most run in. My speedometer does not track correctly from 0 to 100. I used a set speed (drill motor set to correct rpm, 1000 rpm per mile) to check the speedometer head. The faster the drill went the slower the needle showed. Since the gears drive a magnet, which in turn moves the needle, I had to assume the magnet has gotten weak. There for, I calibrated mine to read correctly at 60 mph. Its the easiest speed to set since all the numbers I found on the internet were for 60 mph. At slower speeds it reads a little fast, anything higher then 60 it starts reading a little slower. Milage is dead on since its gear to gear from transmission to indicating numbers.
So when you call for a adapter, let them know how fast or slow it is by milage and speed. At least 10 miles of steady driving on a marked highway is best.
here is some information for you,
http://www.cadvision.com/blanchas/54pontiac/Speedometer.htmlJoe
