I used the torque pro app on my android phone to compare obd speed to gps speed. Obd speed read 2.5 or 3 mph higher than gps speed at 65 mph. Would be a good idea to cross check with 3 other gps laying around the house to verify accuracy, but the readings were pretty constant. That is with oem tire size, but but there will also be difference in diameter among the same size tires across different manufacturers and models. My tires are slightly smaller, maybe .25 inch smaller, only enough to create well under 1 mph difference.

3 mph difference to me is annoying to read 65 mph when my actual speed is 62. To try to compensate all the time is a bit annoying too.

I should ask the question more on truck forums since they change tire size often.
I found a speedometer calibrator by Hypertech that they list as working for a 2004 non-supercharged Buick Regal. That might work for my 2004 Buick Lesabre, but not sure..they are both same engine, but dont know if ecms have any possible differences.. It also costs 200 dollars and apparently different cars use different ways of calculating speed, not just a global obdii thing as i hoped.

So you set your cruise for 68 to achieve 65,,,,I don't see the problem....not worth the money for that much of a difference...