So I know this is nitpicky and stupid but I'm confused.

My 15 Rubicon came with the stock 32s, I have 34.5s on her now. That means that without correction my speedometer should be off roughtly 7-8% and reading too slow. So at 80, I should really be doing 85-86.

Funny thing is that my navigation reads dead accurate to my speedo which means either:
-The speedo runs off the gps...which is silly and probably not the case
-The nav runs off the stock speedo
OR
-I'm terrible at math

Anyone?

 

Your math is approximately right -- We'd need to know the rolling diameters (or revolutions per mile) of the tires to get any more exact.

The speed from GPS navigation is from GPS, and the speedometer is from the PCM (powertrain control module). It's as if someone has already adjusted the tire size parameter in the PCM with a ProCal or similar tool -- Maybe by whoever installed the tires?

As a test, ask someone to pace you in another vehicle and see what speed the other speedometer/GPS is showing.

 

I'm going to test it against the iPhone gps later today

I did the install myself so definitely no adjustment on tire size in the PCM. Very interesting

 

Dumb question, but where on the GPS do you see the speed?? We have the factory 730 head unit and I can't see the speed on it.

 

The speed on the factory jeep Navi doesn't use GPS. It reads off the speedo. Dumbass jeep on this part

 

Thats what was my first assumption, but it seems silly that it would even bother doing it then.

 

I know. When I first lifted my jeep I was like I'm not worried cause my Navi will keep me honest. That's when I noticed it was same as speedo so I asked and was told it's connected to speedo.