I went to 35s a few weeks ago on my JLURR along with a MetalCloak 2.5 lift. Had a shop do the work. When I picked it up I used my iPhone GPS speedo App to verify the change to the speedometer that I expected. I was pleased to find that the shop had reprogrammed the Speedo. Next day, checked again and it was off the expected percentage. Observed this back and forth over a couple of days. Thought the truck was flippin' out. Was driving me nuts. Then I had my wife download a different GPS speedo App and hers was dead on, and mine was off, so I assumed it was the App. Downloaded the same one she was using, and mine was still off, hers was on, simultaneously. So now, I think it's my phone. I then manually calc'd the speed, and the vehicle speedo was definitely off the correct percentage (never reprogrammed) and then it hit me: When the iPhone is plugged into the vehicle USB port under certain conditions (time plugged in, start-up, etc) the Apps both picked up the speed from the VEHICLE, not GPS. So weird, but a warning to all. Unplugged, GPS speed is correct and vehicle is off. Plugged in, they agree and are both wrong. Literally took weeks to figure out.