After slapping on the new tires, I needed to recalibrate my speedo. I know most folks use a FlashCal or a ProCal, both of which are fantastic devices, but earlier this year I picked up an Aeroforce Tech Interceptor gauge to go in the center of my sPOD. This gauge also features two way communications, a feature that I have yet to take advantage of until now. I flipped through the internets and found an older thread, but still a solid read on JK Forum that gave a good description on how to set your speedo for your new tire size. The OP says “The Tire Size is entered based upon circumference in millimeters. Here’s a little chart to make it easier: (You should round the number e.g. 2656.8 = 02657)”
Starting with that calculation pic, I went to the garage to measure the circumference of my tires using a very scientific method… I wrapped a string around my spare to get 2788 mm. When I tried using that setting, my speedo was waaaaaaaaaaay off. On a side note, for those of us with OEM NAV’s, the speedo on the instrument panel and the speed display on the NAV come from the same source. One isn’t a GPS based number.
After quite a bit of trial and error testing, the perfect measurement to get the speedo to line up with the GPS was actually 2643 mm (or the same revs as a 33.125″ tall tire). Either there’s a been a change to how the 2013’s calculate speed and distance (possible, but unlikely) or a tire under weight actually does compress THAT much. Wow! Did I just learn something new?
From the ground up, the half way point (center of the center cap) is at 17 3/8’s. That translates into a (17.375 * 2) 34.75″ true height. Sooo maybe my math is wonky somewhere or I have no clue how it’s properly calculated. The important thing is that the speedo is dead on accurate and so is the odo. I’m just going to attribute the need to use 2643mm to umm… gremlins. Yup, gremlins.