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So I'm changing my engine and transmission on my 1985 chevy c10 and I'm wondering if I need to switch my speedometer, and if I don't will it damage it if I go faster then 85 mph sense it is as far as it reaches?

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  • Welcome to Motor Vehicle Maintenance & Repair! Commented May 26, 2022 at 19:55

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Basically, no, you won't cause any damage to your speedometer. Here's why ...

The speedometers in these pickups used the old style of magnetic sweep (not sure exactly what they are called). This is where the needle is attached to a metal base which is acted upon by the magnetic sweeps. As the magnetic sweep passes by the metal base, it takes it along with it a little bit, which causes the needle to move and show a reading of how fast you are going. As the vehicle moves faster, the magnetic sweep moves faster and reacts upon the metal base more in a shorter period of time, which causes the needle to move more. The speedometer cable itself coming from the transmission doesn't actually touch anything inside the speedometer to act upon the needle. When the speedometer needle reaches the end of its travel, it just stops, but the magnetic sweep can continue to go faster.

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