I recently went from stock rubicon tires on my 2011 JKUR to 315/70/17 Mastercraft MXT. I used spacers with my stock rims. I also did a 2.5 inch Teraflex coil spring lift. Because of the lock pick, my speed would only show 0 mph. Anyway, since the tire change, I was under the assumption I was going about five miles faster than my speedometer was showing on the interstate. I just turned off the lock pick features. I drove it yesterday using the navigation to compare and it's EXACTLY the same. I drove at different speeds from zero to about 75 and it's spot on the whole time. How can this be!? What am I missing? The Nav reads from GPS, right, and not my speedometer...? On a side note, the distance traveled is off. It's basically reading .9 miles on my trip gauge versus 1 mile on nav.