Why not measure and enter the exact loaded circumference and set it to the MM?
I could swear this was in the Edge instros:
1) Set the truck on a flat, fairly smooth concrete surface with a little room to roll. Make sure tire pressure is set to your correct or chosen pressure.
2) Use a large carpenter's square to find wheel centerline of one rear wheel and then mark both the tire and the ground in line with the wheel center and perpendicular with the ground with a magic marker.
3) Roll the truck forward exactly one tire revolution and then use the square again to line up the mark on the tire with the centerline of the wheel and then mark the ground opposite the tire mark.
4) Measure the distance between the marks on the ground and you have the circumference.
This method accounts for tire pressure, tire wear, vehicle load, even temperature if you want to be that pedantic. It will change a few MM with tire pressure and treadwear, I know because I've checked. If you did it with a very hot vs a very cold tire, you could account for that as well.We are talking differences in a few MM. I found a difference of about 15mm between two identical size (but different brand) of tires
FWIW, I find GPS speed readings inconsistent if you are looking for perfection. I use them for spot checks mainly
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