See, there's an interesting thing about tires and speedo "accuracy"... Our speedo's are never accurate. Brand new, 285/35/18's work out to about 25.85 inches in diameter. Thats assuming its measured at full tread depth, which could be an incorrect assumption, but its somewhat trivial in what I'm about to go through...
So brand new, we are looking at about 780 revs/mile. (I realize I'm using the theoretical calulations, and actual numbers may be slightly off, as you can see from the revs posted previously - it doesn't matter though). Since brand new, tread depth is about 10/32 and the tires are "worn out" at 2/32, that means you lose 1/4 inch of tread depth. Overall, the diameter of the tire drops by 1/2 inch, or 25.35 inches. This works out to about 796 rev/mile.
You are making another 16 revs for every mile you travel. Assuming calibration to brand new tires, when your tires are worn, every 48.75 miles you travel will show up with an extra mile on your odometer. This means your speedometer will be off by 2%, which isn't much, but highway speeds, thats around 1.5 mph.
This shows up more as the tire gets smaller, but it won't be much more significant on any tires sizes we are running on our mustangs.
Just an interesting thought. ***DISCLAIMER*** I have no idea how tire pressure and the compression of tires with weight on them effects this.
