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You can adjust it by changing the cable gear in the transmisson. Do some search I think there is a formula to find the right gear needed. FORD parts house usually carries most of the gears. But you may not be able to acheive the right reading depending on diff gear ratio and tire size.

I have a 3.00 with 235/60/15 and I could only get to 7 miles of actual, so I adjusted to 10 miles off to make it easier to estimate. 55 reading = 65 actual
 

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You should probably get a new speedo gear for the transmission with an extra three to four teeth. That should slow down your reading. My '89 had the wrong speedo gear from Ford and it was reading 7 mph too high. I went from a 16-tooth to the correct 18-tooth gear and it's perfect now.
 
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