Doing math on outside diameter you are looking at about 50 feet per mile. That is less than one mile per hour at 60mph. Should not effect it enough that you will even notice

 

The dealer cannot calibrate for a tire size larger than largest factory size. For our Jeeps, that is 255/70/17.

I run 285/70/17 BFG A/T tires, my speedo is off by about 2 mph at 75mph. Doesn't bother me. My "Miles till Empty" works just fine, is not off by any measurable amount.

If you are worried or think you might go over 33" tires, an AEV Procal would be a good investment.

 

I got 34's on 2012 JKU with 373 gears, took it to the dealer to reprogram. They programmed it for 411 gears with 32's and at 70 mph it's only 3 miles off. That will work until Diablo Sport comes out with the tuner for the 2012.

 

Tire Dimensions Made Simple - Discount Tire