Hopefully not a useless post, but found it interesting on our truck and hope others do too.
Just bought a '95 xtra cab 4x4 5speed 3.4 and the previous owner said how the truck was about 5 mph slower than the speedo said. Said it was because he'd changed tires or some such.
Anyway, looked and the truck had the tire size it was supposed to have. 31x10.5 15.
Took it out on the highway and at 65-70 the mph was indicating 3 to 4 mph higher than we were actually going according to the Garmin GPS.
Front tires had been replaced with Firestone Destination AT tires, but the rear were some old other brand tires (forgotten what they were now) of the correct size, that the tread was basically worn off of.
Decided to get a couple more of the Firestones as the fronts were so new so I had matching all around and to get rid of the no tread cracking ones off the back. Measured the tread before going to the tire store.
Front tires 13/32nds depth down from the 15/32nd new depth.
Rear tires 3/32nds tread depth! If my thinking is right then that would be 13/32 - 3/32 = 10/32nds , multiply by 2 = 20/32 or 5/8" shorter diameter overall than the ones on the front.
Anyway, new tires on the rear, drive it down the highway at the same speed and now the speedo is only 1 mph or slightly less faster than actually going according to the GPS. It's close enough to calling the speedo correct now. Just found that interesting.