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Speedometer Gear Question TH 350


JoeShmoe

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I've got to replace the Speedometer drive gear in a Chevy TH 350 automatic because I'm reading about 82% of actual speed because of oversize tires and wheels. Do I need a gear with more teeth or less teeth? I've got a 22 tooth gear in there now. Any suggestions? Thanks in advance...Joe G

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If you know the tire size and rear axle ratio, then a motivated GM parts person might be able to find the original drive/driven gear set for a similar factory combination. I've been able to do that in the past. In some cases, a ratio adapter box is used . . . some are factory, yet some speedometer shops used to build their own "custom" ones.

BUT, in order to correctly do the deal, you'll FIRST need to know how accurate the odometer is reading. The odometer is driven directly by the speedometer cable. The speed reading is the result of a bar magnet spinning (at speedometer cable speed) and then causing the other speed cup to move, which is attached to the speedometer needle and counter-acted by the clock spring which makes the needle return to "0" when the vehicle is stopped. The magnetism can be varied until the speed reading is "in spec", so it's variable whereas the odometer reading is not. Hence, the need to check odometer accuracy FIRST.

What's the tire size/axle ratio combination?

NTX5467

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That's ONE great speedometer gear/cable website!

In using some of the formulas on that website, you can go into the TireRack's website and find a tire size like you have, then check the specs page for "Revs per mile", and plug that into the formula to get the "Tire Diameter", which can help define the particular speedometer gear pairings you might need.

Over the years, I'd seen almost all of the GM items on there, but not the other brands' similar items.

Thanks for that information!

NTX5467

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I think "they" make gear assemblies that mount external to the transmission that are easily adjustable.

The "ratio adapters" have specific gears inside of them for specific applications. Nothing "adjustable" per se, just change the inner gears' gear ratio.

The noted website has adjustable "box" listings for electronic speedometers, not unlike how you reprogram some of the factory systems when tire sizes are changed from "as produced". On some of the later 1990s GM pickup speedometer electronic clusters, there was "a resistor" which plugged into the rear of the cluster, from which you removed particular "pins" to get the desired value for the particular tire size, so the speedometer and odometer would be correctly calibrated.

Enjoy!

NTX5467

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Thanks for the help, guys, especially the link to the PATC site! Great information.

I did get my original , most basic question answered by another source (HAMB). I need LESS teeth on the DRIVEN gear, if the Driving gear remains the same. Thanks again for the help!....Joe

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