My experience with the HyperTech has been good. It reclaimed some of the lost fuel mileage close to stock, but taught me tire humility.
I keyed in 36.5" for the diameter of my KM2 37 x 12.5 x 17 tires and that gave way too high of a speed. So I went back to the chalk method of measuring the distance of one complete tire revolution. Result was a diameter of 35 1/8" in spite of the tire claim of 37". All this time I have been bragging that "I have 37's - haha". So now I hang my head some and humbly answer back "Ya, they are 35's".
Good news is that the speedo is dead-on.

 

My last experience with built in NAV/GPS was about 10 years ago when all that tech was still in its infancy and I remember something about it having a wire(s) that needed to be connect to the vehicle speed sensors. So I'm guessing by the responses here that the technology has now progressed where that's no longer the case (no vehicle speed sensor tapping).

 

Another way that many on JK-Forum have used with great success is to measure the height of the tire from the ground to where the tire shoulder is--not to the top of the bulged out tire, but to the point where the tire makes a sharp bend to the tread. That's what I did and my speedo is dead on (with 33s that measured at 31.25").

 

Do you have to buy one of those reprogramers? I had thought the dealer will punch it in for you for a fee?

 

Took mine (with 37's) to the dealer and they programmed it for "255/75/17" tires, which they claimed was as large as they had in their computer. So they re-programmed it to EXACTLY what it was when I dove it in, then tried to charge me.
Point is, buy the reprogrammer.

 

Just buy a ProCal and call it good. Besides that fact that is fixes the speedo, it will also corrects shift points if you have an auto, and corrects the timing for your safty features like ESP, airbags, anti roll, etc.