My JK Sport (non-S model) came with the tiny 225/75R16 Goodyears. I'm about to put on a set of BFG Mud Terrain 255/75R17, which of course is the stock Rubicon set up. Here's my question: Do the folks at Jeep really do a different speedometer calibration between the two different stock tire sizes?

 

Absolutely

 

Yep, take it to the dealership and ask them to adjust it... Or of course but a programmer.

 

Yes they do, and yes should also do it.

 

I have a Flashcal on the way. My question was really just aimed at wondering if the Toledo plant actually took the time to calibrate between the two tire sizes.

 

Good question, I did the same thing, Rubicon take-offs ; I "assumed" that there would be no re-flash necessary for the largest "stock" tires from the factory. Meaning, the computer would adjust for what is a factory size.
So I have to Flash for correct speedo, mpg readings ? Manual trans, so shift points not an issue as with auto.

 

With the rubi tires you will be off about 6mph. So when your speedo says 65 you are really about 71 mph.

 

Here's the difference between the two sizes...

 

Factory would have outfitted the Jeep for that tire combo for sure = you will prob be off if you don't adjust.

Get the Flashcal, and make sure to do an actual measurement of the tire and enter that value - IIRC the stock Rubi setup comes in at 30.25" in the real world, but don't quote me on that one.

Confirm it with your phone or GPS after setting the flashcal, and you are good to go. Of note, I am pretty sure any changes to tire diam. in the tuner will override/reset any power level changes you make (again, this is from memory so I may be wrong on that - just recall reading it somewhere). Do some searches for both topics as the info is out there, in either case - the flachcal will be a nice mod for ya! (running a traildash here and like the 87 tune myself, runs better than mileage XS did and gets better fuel economy too!)