3 questions

[Jackpine]

AND, I think you corrected the wrong way? You said you recorded 25.5 km on the dash, but the GPS said 24.6? This tells me your "entered" Tire Size (TS) is too large. The PCM "thinks" you've traveled farther than you really have, because your entered circumference is larger than it should be. Notice your Gryphon and your dash are saying you are traveling FASTER than your GPS is saying, which also correlates to using a TS that is too large.

But, now I have a bit of a problem. Your tires are 33x12.5x20 ones. These work out to 33 x 25.4 x pi = 2633.3 mm without reducing for "squish", which is even smaller than the number you were apparently using before (2661).

If I apply the correction to 2661 properly, I get: 24.6/25.5 x 2661 = 2567mm, which is the number I think you should enter to make your Gryphon and GPS agree - the truck's speedo will still read about 3% FASTER - about 82.5km/hr (indicated) at 80km/hr (actual). And, the 2567 corresponds to a "squish" of about 2.5% which sounds right for your low-profile tires.

And here's the rub - In ALL cases, your speedometer was telling you you were going faster than you really were, yet you got tickets for speeding! Were you "indicating" 100km/hr in an 80km/hr zone? Do the police have you on their "hit list"?

Things don't quite add up. However, to get your speedo where it should be, try 2567mm.

- Jack