Speedometer off by 3-5 MPH?

1984 CHEVROLET TRUCK
170,987 MILES • 7.4L • V8 • 2WD • AUTOMATIC
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TWEBB101
  • MEMBER
  • 183 POSTS
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.
Feb 21, 2024 at 9:02 AM
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STEVE W.
  • ✓Experienced Mechanic
  • 15,306 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.
Feb 21, 2024 at 3:18 PM
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