[Dodgevanman]

It's probably something internal in the gauge itself. I think the odometer runs off of small gears in the speedo head.

 

[Dodgevanman]

Does your truck still use a speedo cable or is it electric via a speed sensor?

If it's a cable, I'd say lube the cable. If it's electric, you may have to replace the speedo gauge.

 

[Dodgevanman]

There's a way to turn the mileage up or down (hey used car salesmen do it! LOL) but I'm not sure how. I would assume you would have to dismantle the speedo head to gain access to the numbers.

If your a law abiding citizen..a call to your local MVA might help. They may have some information on what you can do to certify your mileage.