Originally Posted by weldingbob2
I have a question, not sure if this is the right thread but here goes. My son is going to order a base JK in the spring when hyper green becomes available. He is going to buy some 17 inch take offs from Craigslist. How much will this affect the speedometer and is that something that can easily be adjusted?
The base Sport tires are 225/75R16, which is a little over 29" in diameter.
Depending on which model your son gets his take-offs from, they range from 245/70R17 (30.5") on the new 75th Anniversary Edition to 255/75R17 (32") of the stock Sport S, Willys, or Rubicon.
Let's use 255/75R17 for comparison since they're the most available.
It's about a 3" difference between the base Sport diameter and most Wrangler take-offs. That makes for a 9.5% difference in diameter or rolling circumference. Most tire places, and advice on the Net, will suggest a diameter departure of 3% or less to avoid issues.
This is a good website for calculating the differences between tires:
The two main issues with changing tire size without recalibrating your Jeep's computer to recognize them is having a speedometer that's reading fast (or slow) and your odometer will not reflect accurate mileage.
If you don't program new tire size, it can also mess with an automatic transmission's shift points. Not a problem with a manual gearbox, but if you get an auto, and the computer doesn't know bigger tires are installed, it can cause the shifting to get wonky.
Programming a Wrangler is a simple process if you're willing to pay an extra ~$160 for an AEV Procal
or Superchips FlashCal
. Or, you can ask your dealer to program the new tire size. Some will agree to do it as part of the sale, or charge a fee (I've heard as low as $50, and as high as what a programmer would cost you).