WTF! Recently added a bigger sized tire to my truck. Had a tech hook up WMDS to adjust my speedo, and I cant adjust it. He also said he encountered a F-150 that he couldnt adjust the speedo one. I talked to a performance tech(whack job) and he said I had ot buy a $400 performance tuner to adjust my speedo. What do I do???

Wandell, I'm looking at you

 

On the 97-98's, the speedo is not computer controlled, it's gear driven. You have to adjust it with a different size plastic speedo gear.

 

{thinking chevy wise}.... Is this plastic gear in the tailshaft? How many MPH am I off by? I went from a 255/70/16 to a 285/75/16. I drove past a police monitor the other day, I was doing 35 and it said 38, so is a 3mph difference all I have?

thanks man

 

I'm not positive of the location. Your speedometer is off more the faster you go. I went from and to the same size as you. Mine is off 5mph at 60mph.