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1950 Buick Speedometer Calibration

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Thanks to everyone that responded to my post on  speedometer removal. I successfully removed the speedometer, took it apart, cleaned all the old hardened grease, and reassembled it. I also replaced the cable. The good news is that I now have a functioning speedometer, bad news is the speedometer reads 20 mph, and the speedometer app on my phone says I'm going 32 mph. Since the speedometer wasn't working when I got the car, I have no way of knowing if it was accurate before it stopped working. I was wondering if I did something while I had it apart to cause the problem, or maybe, and I doubt it, the speedo gear in the trans is incorrect. Since I'm now pretty good at removing, and replacing the speedometer, how do I calibrate it? I read somewhere it may have something to do with demagnetizing the magnet when I had it apart.  As usual, any and all help is much appreciated. 

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The first thing to do is to check if the gearing is ok. Gearing is a separate issue from speedometer calibration. Take it out and drive it 10 miles, while paying close attention odometer and to the mile makers. If you gained a tenth, the speedometer is geared 1% fast. If it gains a mile, thats 10% fast, etc.

 

Gearing is fixed with the little set of speedometer drive gears in the transmission. The steps (for different gears) are usually around 5%. A different tire size can also make a correction.

 

If the speeedometer still reads wrong, then it needs calibration. You could drive 60mph (one mile a minute) by the speedometer for 10 miles, and have a passenger time you with a stopwatch. It should be exactly 10 minutes. You can caluclate how much change you need to make from there.

 

I don't know about the magnet. I usually get the gearing right and let a speedometer shop calibrate the head if it needs it (it usually doesnt).

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