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Posted by David Sherman on Thursday, June 26, 2008 at 11:50:51 :
In Reply to: speedometer calibration posted by Bob in Indy on Thursday, June 26, 2008 at 08:28:28 :
It seems to me you'd have to know what speedo drive RPM is supposed to be for a given speed indication. Maybe all the big rigs at the truck stop are the same (same gear ratios all the way to the axles and same tire diameters) but I wouldn't rely on it unless I knew that, say 1000 RPM on the speedo drive was supposed to make it read 50 mph, or whatever the numbers are. I rebuilt the speedo on my beater M37 and calibrated it using a variable-speed drill to spin it while I adjusted the spring. It seems about right driving down the road, but I can't remember how I figured out what the correct RPM/speed ratio was supposed to be. Perhaps there's a standard ratio that applies to all speedometers. I just don't happen to know what it is. I'm pretty sure all military tachometers read twice the RPM of the actual drive cable. If they were built with the exact same mechanism as the speedometers inside, then if an M-series tachometer reads 4000 rpm full scale with a 2000 rpm drive speed, an M-series speedo should also read full scale with a 2000 rpm drive, and the civilian dodge ones should be the same. That's a big "if", though.
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