You can crack open the speedometer and adjust the clock spring inside. Tighten the spring, the speedometer will go slower. Loosen the spring, the speedometer will go faster.

The factory uses a little drop of paint to keep the spring from moving after they calibrate it. I used a drop of fingernail polish.

The way I calibrate my speedometer (my odometer was reading about right-> within 3% or so, and the speedometer was reading way to fast), remove instrument box (I built an electric temperature gauage, so I didn't need to remove the mechanical one, also be careful of the live wires touching ground-> tape them up), open up instrument box (the chrome is pinched on both sides, so pry open the box and remove the speedometer face -> some tiny screws), set the instrument box up so you can get to the clock spring ( I posted pictures of the clock spring a while ago, I can repost them if you want to go this route) and be able to put the speedometer face back on to see were the "ribbon" is reading. Ideally, three people and two cars would be the best way to do this-> One person/car (with an accurate speedometer) driving a certain speed next to the car you are setting the speedometer, the car you are setting the speedometer -> one person driving, and the other person adjusting the speedometer. After you think the speedometer is set, check it against different speeds with your pace car. This take a while, but I got my speedometer within 5% of right speed.

Paul