91 F150 Speedometer
Post Fiend
Joined: Nov 2002
Posts: 9,987
Likes: 7
From: Memphis, TN
91 F150 Speedometer
So it's reading 25% high. What's your original tire size and current size? Are the gears the original ratio? Has the trans/t-case ever been changed/repaired/modified?
The speedo uses spinning magnets to push the needle against a spring - the faster the cable spins, the more it's able to compress the spring and push the needle to a higher number. Gumming it up would show a lower number than it should.
The speedo uses spinning magnets to push the needle against a spring - the faster the cable spins, the more it's able to compress the spring and push the needle to a higher number. Gumming it up would show a lower number than it should.
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Senior User
Joined: Sep 2001
Posts: 291
Likes: 1
From: Central Texas
91 F150 Speedometer
Runnin BFG 255/70/15's which in my estimation is close to the same diameter as 235/75/15's which are the stock tires. It just started this a couple of weeks back (no mods)
Post Fiend
Joined: Nov 2002
Posts: 9,987
Likes: 7
From: Memphis, TN
91 F150 Speedometer
I dunno - maybe there IS something in there between the spinning parts & the needle, like a spider web or a flake of rust. Pull the speedo and take it apart. If your GPS can run off the truck power, you can use it as a temporary speedo until you fix/replace yours.
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