[Longshot270]

Yes, make sure your tire size and gear ratio is correct after you load previous settings.

I also found that the easiest way to get the tire size close to actual is with a simple conversion using the odometer (higher accuracy than the speedometer). What I do is drive about 10 miles after clearing the odometer on the truck and GPS.

*Make sure the GPS has been able to get an accurate location otherwise it will say you are travelling when in reality you can be sitting still. That can be a real problem because the distance you never traveled is recorded on the GPS odometer.

Truck distance(truck odometer)
Actual distance(GPS odometer)

...multiply the tire size by whatever this value is.

Here is an example of how it should look.
Truck odo: 11.0
GPS odo: 10.54
Programmed TS: 2410

Math: 11/10.4=1.058 (so it means your current TS is about 6% too small, a real difference when the value is in mm)
2410 x 1.058=2550 after rounding

It may take a few tries if you are using short distances but you will get close eventually. The further you drive the more accuracy you will get, but the short 6-10 mile trips will work well enough to get within a decent % error.

And just to add, the tire size calculator in pegasus is very helpful in getting a close size in the beginning. Jack is probably right with the 3% squish on stock rims but wider rims+stock tires (like mine) are around 4.5%