Well, Dana got her ticket for it the other day: going faster than her speedometer read. Well it was actually for driving too fast- paired with going faster than her slower speedometer read. Anyway, she decided it was finally time to fix her speedometer to correct for her non-stock tire size of 33's. I had also been pulled over on the freeway before for the same reason in my old truck but I played dumb and asked the officer if it was true about my new larger tires messing up my speedometer. He ended up letting me off and told me to take the $250 he just saved me to a speedometer shop to have mine properly calibrated.
Now with a Jeep Wrangler, correcting for tire size can be a simple fix by switching out your speedometer gear. Easily accessible, it is located underneath the speed sensor near the tail shaft on your transfer case. Basically by switching out the original gear that was installed to match your stock tire size and stock gear ratio, for one that has a different amount of cogs, or teeth, you can reset whatever your modified tire/gear set up currently is.
The correct gear can be purchased at your Jeep dealer and the teeth count and part number can be figured out with a simple chart:
To arrive at a correct Jeep® part number add the tooth count for the speedometer gear to the end of 520676. For a 26 tooth gear the correct part number is: 52067626.
* Items in bold have been verified by person(s) with a GPS to be within 3 mph of actual speed at 70mph.
*chart obtained from Raingler.com
We figured out that part number 52067631 was the correct gear for Dana's set up and she was able to pick it up at the dealer for under $20. The difference in gear size is quite apparent, moving down in size from 34 to 31 teeth.
We decided to gather our own data on her speedometer too, before the switch-out, to see how well the fix worked, so we hopped in the Jeep pre-install with my GPS to take some numbers down.
We picked various measurements of speed then hit the streets and highway for our "scientific" tests. After achieving our set speeds as displayed by the Jeep's speedometer, we took three readings from the GPS and used the averages for our final data input. We would do the same thing after installing the gear to see how close we had come to true MPH.
After we had driven all of our predetermined cruise speeds and gathered all of our info, we headed back home for the actual gear install.