Speed issue

It's most likely saving your employer money on fuel.

 

if the computer is hooked up he can see how fast you went all day
when you shifted and stopped

 

Does the speedo say 65? If so, get the make/model/size of the drive tires. Most tire manufacturers publish revs per mile on their website, get that number. Then you have to hook it up to Insite and verify that the revs per mile are correct, if not, correct it.

As far as proving to your employer, all you can do is run convince him to ride with you and run a set distance and show him that miles are racking up faster than mile markers. I'd do that myself first just to double check the GPS. My GPS is a few miles per hour off too, but the miles show correct on it. I do not know the algorithm they use to figure speed, but it may not be perfect. Double check using mile markers, then raise a stink.

 

whats does the speedometer say? Most likley the tires are smaller or worn. If the truck thinks it going 65, it wont let you go faster, even if its wrong. GPS speed has nothign to do with axle speed and thus is more accurate.

 

I don't think that you have a bad speedo sensor becasue it sounds very consistent, they do not fail like that. It's very odd that the speed is 3 MPH off at all speeds though. Usually they are farther off the faster you go. 3 MPH off at 50, would be about 1.5 MPH off at 25 MPH. No mater how wrong a parameter is set, one speed that will always be correct is zero MPH, then it spreads from there, the faster you go, the more it will be off.

Anyway, verifying parameters is more than just looking to see if it set to 65. Gear ratio and tire revs per mile are the big factors to look at. Get the tire brand and look up the revs per mile. Have them verify that it is correct.

For the recorod, we have about a 4 MPH spread on our 255/70R22.5 drives. Brand new, the actual speed is about 2 MPH faster at 75 MPH. At 3-4/32" (ready to get pulled), actual speed is about 2 MPH slower at 75 MPH.

3 MPH slower at 65 will taller rubber is not terrible if your tires are close to needing to be replaced. You will gain 4-5 MPH with brand new rubber. Cummins does have a tire wear factor that can be programmed, but I have never seen anyone actually use it. You may be able to talk you employer into setting it up. This will keep your speedo more accurate throughout the drive tires life.