[mickmate]

Yes I have done this and it worked quite well. Did you look into different gears? I'm guessing that would be a simple proportioning problem. 70/90 = .777. A 14 tooth gear X .777 = 10.88 so you'd use an 11 tooth gear if you could get one. Try turning the screw but make a note of where it started, both on screw position and needle reading. There are a few good articles from a google search on speedometer calibration and adjustments depending on the principle. Some of us aren't afraid of finding out how things work and tinkering with them a little, go for it (in a methodical way of course). Let us know how it works out.