well i guess i need to adjust my speedometer in my 91. i got a fast driving award last week, and was informed by the officer that i was goin 64mph, when his radar hit me, i looked and my speedo said about 57. i checked it with my GPS, and well at 57, im goin 61. whatever the case i need to adjust it. does anyone know how to do this...if its possible?? any help is appreiciated

 

IM not sure what years started to use computer controlled speedo readins, might be 91' but I kno on my 82' i have to replace the speedo gear thats at the top of the t-case. Theres some formula to calculate it, I have it in my JBG mag, but thats in the truck right now. Good luck man

 

Ford didn't start using the PSOM (Programmable Speedomoter/Odometer Module) until '92. You will have to change the output gearing to the cable. Out fo curiosity, what has been changed on your truck that would put the speedometer +4 mph off. Larger-than-stock tires or something else?

 

mine consists of 3.70 and 33's

 

just over sized tires...32x11.50s, prolly next summer ill be getting one size smaller tire, 31x10.50. but thats the only change, still have stock gears 3.55:1

 

That'll do it.

I'm running 33's and 3.55 gearing. MPH = I don't know - my speedo is broke. I'm guessing I'm about 10 mph + or - a few over the stock set up. At 1500 rpm's I'm running around 70.