Help with Speedometer adjustment.

how worn down are your drive tires when my tires get about half worn it will be about 3 mph off. that isnt really that far out of wack. if that isnt the problem then you can readjust the speedo. when i reratioed my truck i had cat adjust the speed in the ecm. yours might be the same way.

 

you could have it checked but i will bet there isnt anything wrong with it all speedos are not 100% accurate 100% of the time.

 

heck look at it this way at least when you think your doing 68 you are actualy going 65 no tickets that way!!

 

Actually as you described it, the miles driven would be the opposite. If speedo is reading 68 you would do 680 miles in 10n hours, but actual is 65 so 650 miles in 10 hours.

Some companies set their trucks up this way...to save 10% on the excess miles on a lease.

 

didnt think about that. does that mean if i readjust my speedo i could save on logbook hrs?

 

That would be opposite again....and actual miles is figured by odometer or hub meter.

And in answer to the earlier question...the speed/miles is in the ECU, if you are off 5% it means that the tire size is wrong in the ECU set up...any dealer can adjust to get accurate tire rotation to the speed and distance traveled.