As I recall, the basic principle of the thing is that there is a round magnet that spins with the speedo cable. It attracts a similar size piece of steel through an airgap... said piece of steel is attached to the roller on which is the orange "ribbon". Holding the roller back is a very lightweight spring... so teh speedo can't spin as fast as the magnet does.

You *might* be able to increase the airgap between the steel on the roller and the magnet, but not enough to make a 20mph difference. I'd guess that the hold back spring is broken or weak. In either case, you gotta pull it out, and take it all apart. Which is actually fun, but it sure wouldn't hurt to have a spare on hand in the event that something goes terribly wrong.

Or you have the wrong gear in the transmission as has been mentioned. Or you have the wrong gears in the rear end.

Does the odometer seem accurate? If so, the problem is in the speedo. If it clocks off miles faster tahn it should, then the problem is in the gears somehwere. Is it off by a consistant 20mph? Or is it more of a ~30% too fast reading...?

I'm glad you brought this up though, as my speedo also reads fast, and you got me to thinking about it.... Maybe I'll look into mine. though probably not.

-Matt
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-Matt '70 145s, '65 1800s, '66 122s wagon, others inc. '53 XK120 FHC