Quote:
Originally Posted by markeb01
I'm not sure changing tire diameters is the best approach to speedo accuracy this many years after production. Too many thing may have been changed over the life of the truck. The ratio adapter noted above may help, but it might not be the ratio change you need.
A speedometer shop would be the best bet as they can simply change the plastic driven gear in your transmission to one that gets you very close to a correct reading:
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What I did was get and mount a GPS on my windshield showing speed and then doing as Mark suggested using different gears [teeth count] till I zeroed it in. BTW my trans is a TH350
Bob