Keep in mind that the on board computer and odometer are calibrated to the mfr recommended tire sizes (and make, to a limited extent). I changed sizes from 245/40 to 255/40, and added 2.5% to any distance traveled.
For me, thats 50 miles, over 2 gallons of gas, per tankful. My odometer still says I'm getting about 250 miles per tank.
Also, odometers have a range within which they will pass QA/QC. I think the figure is plus or minus 5%, but I can't remember for sure. They may or may not be accurate. The on-board computer readouts are usually more accurate because they use a separate pulse counter from the odometer, and do not suffer from being converted to an electronic signal to drive the needle.
Some of the newer autos will allow you to tell the computer what tire size you have on your vehicle so it can compensate.
All of this is to say that, comparisons between whatever changes you make with your vehicle will result in a percent change that you can actually use. Whereas any comparison between vehicles, even litter mates, is a crap shoot, at best.

Isn't the certainty fun!

BTW, using about 30 miles of roadside mile markers, my odometer was off around 4% before I changed tire sizes.

Kirk