[MTAS]

Originally Posted by 28HopUp

Because wrapping a tape measure around the tire and dividing that into the measured mile is NOT the correct way to calculate the tire revs per mile. A tire's contact patch will throw off the calculation, so you have to use the manufacturer's published number. If the tire company doesn't publish a number for your particular tire, then you are better off using the number from a different company for the same size tire (not the best choice, but better than guessing).

Using 157db's method will result in your speedo being off. But here's the question - If the correct number is 770, but you plug in 740 into your handheld tuner, will be speedo be reading faster or slower than you are actually traveling? I know the answer, but I just wanted to give you something to think about this morning.

That kind of thinking makes my head hurt and I even took physics in college but that was a LONG time ago I completely agree with using the manufacturer's web site as a reference when changing the revs/mile with a tuner. FWIW, I recently switched from Nitto 555 295/45/18 to Toyo Proxes ST II of the same size, and the revs/mile each manufacturer listed on their web site was different (only by 1 rev/mile, but I want the speedo to read as accurate as possible), so I adjusted accordingly with my Predator