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Speedometer off by 3-5 MPH?

Tiny
TWEBB101
  • MEMBER
  • 1984 CHEVROLET TRUCK
  • 7.4L
  • V8
  • 2WD
  • AUTOMATIC
  • 170,987 MILES
The speedometer was reading 5 MPH slow, so I replaced the driven gear with a green 42 tooth gear. I pulled out a yellow 41 tooth gear. The reason I changed it is because the tire size changed from factory, and according to TCI’s calculator, it should be 42 tooth now. After test driving it, it still reads slow, about 3-4 mph slow. Obviously, I can live with that, but at this point would the issue be the speedometer itself? Or something else? I have also replaced the speedometer cable as well.
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Wednesday, February 21st, 2024 AT 5:02 PM

1 Reply

Tiny
STEVE W.
  • MECHANIC
  • 13,509 POSTS
Normally I set them up to read a bit high. That lowers the chance of getting a speeding ticket because you are going slightly slower than indicated, that is how most factory set ups are as well as that allows for slight tire size differences but keeps you "legal" I suspect you could switch to a 43 (purple) and get your reading 2-3 over.
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Wednesday, February 21st, 2024 AT 11:18 PM

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