I was using my GPS in my Dodge and noticed that the speedometer was off in the truck. THe GPS was saying I was going 65mph, but the speedometer read 70mph? ANy ideas on what could be causing this? The tires and driveline are stock, no oversized anything.
My only guess is the rear wheel speed sensor is bad?

 

most of the time there is a slight speedo error but 5 mph at 70 is too much. is there a possiblity that the previous owner had the speedo calibrated for a larger tire? If you put larger tires on a truck or car, the speedo will be lower than the actual mph, your's being faster tells me the PO had larger tires on there...

 

I doubt the PO had larger tires, he was a friend of mine. The first owner I doubt had larger tires as well, he put most of the 209,000 mile on it and I don't see that happening on anything much larger then stock. On the other hand, the wheels on the truck seem to be in too nice of a condition for the mileage and two of the tires have what look to be that junk yard writing on them (yellow pen marks for the part ID).

OK, how would adjust the speedo to be accurate?

 

you'll have to find a speedo shop or take it to the dealer. your truck may still be using the gear in the tranny method (not sure on this, you'll have to ask the dealer...). My truck has a speed sensor in the rear diff. if yours has one, it'll be on the top of the diff with a couple of wires coming from it and it's easy to swap out, but be careful not to break the new one on the install, it's fragile (mine broke on installation).

209K miles... [sm=hail.gif] how's it run?

 

I was thinking of replacing the sensor on the rear diff, I hear it's like $25. BTW how do you like the MSD ignition, I was thinking about one of those and the FIPK from K&N.
Thanks

 

I like it, 1st thing I noticed was the motor reved easier. to me that means less un-burnt mixture and equals a tad more power. couldnt tell you if I get better milage or not, my plenum is leaking and killing my power. the rear diff sensor from the dealer was $35 bux.