My speedometer has been reading about 13mph over the actual speed I'm traveling. My dealer says I should spend $900 for a new harnes or cluster? All other guages work just right and when I do the instrument cluster test all is well except one error code 66 All door lock output circuit shorted to ground.

I've given up on my dealer! Will figure it out with a little help.

Thanks

 

What year vehicle?
Oversize tires?


phox

 

2003, stock tires, I've tested the speed with a gps and I have another identical truck that speedo works just fine.