the only problem with doing all this math is that actual tire sizes are almost never what the rated size printed on the sidewall says. in order to get concrete numbers, you have to take into account the static radius measurement. this is the distance between the center of the hub and the ground. you multiply that by 2 to get the measurement that is more accurately read by all the spinning parts of the drivetrain as your actual tire size, including the speedometer and odometer. then you do these calculations using that number. don't be alarmed, it will be much smaller than the stated tire size, but that's due, in part, to the slightly flattened footprint due to the weight of the vehicle, unless you have about 100 psi in your tires.

this is still not accurate if your tires are low because then you have to take into account the circumference of the tire still has to make it all the way around for each rotation, but the static radius won't give an accurate depiction of the circumference for a low tire. so then you get into all sorts of friction dynamics, because, really, there are times during the rotation of a low tire where the ruber is being dragged or pushed on the ground due to it slightly wrinkling and just not being stretched as much as it would be with the proper psi.

then again, proper psi is not the min and max labelled on the sidewall. it's usually much, much less as far as load-carrying capacity goes, but people's driving habits go from a leisurely trip to the store to hauling ass to get to work on time and if that sort of stress is put on a tire with a comfortable psi, the tire will fail, ala Ford Explorer.

done.