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I was wondering about this I know they say used car dealerships used to do this all the time. I replace the gauge cluster in my 77 so the miles don't match the truck anyway and since I just rebuilt the motor and it has maybe 500 miles on it, I was wondering if anyone could tell me how to do this so I can have the odometer match the engine. Just curious?
I did this when I had the old guage cluster in. My method was kind of goofy but it worked. I pulled the speedometer itself out of the cluster. There's a shaft of sorts that the odometer numbers roll on and some sort of bracket mechanism that holds them on there. I did a little bit of prying and was able to get just enough slack between the number dials that I could slowly turn them one at a time. It was a very tedious project (been a while since I did it) that took me about an hour to do but when I finished, and got everything back into place, I was reset to 0000.0. If you do this just be patient and be sure you don't force the dials. When there's enough space between them they will roll freely so take your time.
I've heard of people using drills to slowly turn the odometer back, but I'm not sure how long that could take and I'm afraid of using too much speed when doing this and damaging the speedometer or at least goofing up its calibration.
Opening the speedo head is a good way to ruin it. A tiny spec of metal can mess up the magnetic clutch unit. The speedo heads won't turn backwards unless they are on a very old vehicle.