Q&A / Tech

Monday Mailbag: Correcting Speedometer Gears for Accurate MPH Readings


You’ve got questions. We’ve got answers — Mondays, when the Summit Racing tech department tackles your automotive-related conundrums. This week in our Monday Mailbag we’re helping a reader find the correct speedometer gears for accurate speed readings in his 1970 Ford Mustang.

Q: I am having trouble finding the correct speedometer gear for my 1970 Mustang. The car has a top loader 4-speed transmission and a 4.10:1 rear-end ratio. I’m running 26-inch-tall rear tires, and a 21-tooth speedometer driven gear. The 21-tooth gear is the largest I can find, and the speedometer reads about 27 mph fast. Do I need a special reducer?

A: Based on the information you’ve provided, it sounds like you have an 8-tooth drive gear in the transmission. You can change the drive gear to a 6-tooth gear and use a 19-tooth driven gear to correct your speedometer reading. If you’re unable to find the correct combination of gears of however, you can have a custom reducer made at a speedometer shop.

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  1. Would you share the math behind the advice? I am considering swaping my car’s tires for slightly bigger size and that could come in handy.

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