Self Taught Tuning : By Mooseditty17

You don't say what your tire change was.
Myself I started with the 30.6" diameter stock tires and moved to 33.1" diameter tires, a 2.5" difference. With the original factory tires the speedometer and cruise controls were the most accurate of any vehicle I have ever had. With the larger tires I have to set the cruise control to 61MPH to achieve an actual speed of 65MPH. Goodyear Wrangler All Terrain 265/70R16 to BFGoodrich T/A KM3 255/85R16.
Note: Actual tire measurements came from Tire Rack website using the actual make of tire/size for comparison and speed was/is verified by two GPS units. One of those GPSs can measure down to the tenth of a MPH. Steady speed was never more than the 1/2 mile off due to rounding.

Why do we have different scenarios/displays? Does your needle agree with the digital display?

 

Went from stock 265/70R16's to 285/75R16 Duratracs (33.1"). When I programmed the calibrator I measured tread depth, subtracted 1/8" (end of life) and divided by 2 to find the half life of the tire...I think 32.75" was what I entered for tire size in the calibrator. That way the speedometer would be slightly off at first, dead on at mid tire life, and slightly off and end of life instead of being dead on at first and just continuing to get farther and farther off as the tire wore.

Prior to the calibrator my truck would display 70mph at an actual speed of 75mph with the 33" tires. After installing the calibrator with the way I programmed it I now display 75mph at an actual speed of 73mph. At half life it will be dead on and then only 2mph off at end of life, compared to being dead on now and 5mph off by end of life if I had programmed it for the full 33.1" new tire size.

So, back to my last post, at an indicated 75mph I'm actually going 73mph and the cruise control displays 74mph.

 

So, we both started with the same tire size and went to the same diameter of tire size according to Tire Rack's info, I looked your tires up. You have installed the speedometer corrector, I have not because of one reason. That reason being that I have heard of it not correcting the Cruise control speed from many posts here on TW. However, your readings of only two MPH difference seems acceptable, but a mystery.

Hijack over.