Chevrolet Speedometer

[gordon]

If you think about it Stew, the speedo guts need to do something before the needle moves off the bottom stop (and this applies whether or not there is a stop pin at the zero point) This could be a lot or a little, certainly enough to change the speedo reading by 20mph or so.

The correct way to calibrate this sort of instrument is to spin it at the speed representing half the full scale deflection (say 30mph) and put the pointer shaft on while it's turning at that speed, then change the speed to 25 and 75% of full scale deflection (15 and 45mph say) and check that it's accurate at those speeds.

Since we don't know the speed it's supposed to be turning at, and we dont have calibrated speed sources to hand anyway... taking a stab at the mid range and checking that it's reasonably accurate should be good enough for our purposes. The legal situation in the UK is that any vehicle that isn't pedestrian controlled has to have a speedometer accurate to +/- 10%