My father bought an 98 F-150 50th Anniversary, and the previous owner changed the gear set for towing. The speedometer is off by about 5 or 6 mph at highway speeds.

I am wanting to purchase a new Diablo chip for my dad so that the speedometer will be correct, but I have no clue what gears are in this thing?

I would like to hear everyone's creative ways of helping me figure this out, either mathematically with gear ratio of the transmission numbers or easy ways to count the driveshaft spin while the tires roll one time or whatever you guys can think of.

Thanks!
James Ayton, Jr.

 

Also, could the superchip that is in there now be reproggrammed to fix the speed-o?

Thanks again,
James Ayton, Jr.

 

are 1998's not still a nylon gear? I didn;t think they started using computer programing till 1999. for my 1998 I just have to buy a little gear to corect the speedo.

Steve