I once tried to turn back the miles on the odometer and it broke. the needle still worked but the odometer didn't I replace the old speedometer with a junkyard one and it all worked again.(It wasn't a chevelle, but thats not important.)
that's all I could tell ya.
If you want to tackle this yourself, it's not too hard. You will need to remove the speedo from the car and disassemble it and replace the odometer. Otherwise take it (either remove the speedo and take it or take the whole car, which will be more expensive) to a good speedo shop.
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