Okay...I am thinking my Jeep may be smarter than me I just upgraded to a set of 35x12.5 Fierce Attitude tires (love these tires by the way). I expected my speedometer to be off by between 9 and 10 percent. So the first time I got on the highway, I quickly cross-referenced the speedo with the speed reading on my factory navigation, and the two are still reading exactly the same.

I originally assumed the navigation system computes speed via satellite, but how could that be possible if they are reading exactly the same both before and after switching from 32's to 35's? Is the speed reading on the factory navigation system somehow connected to the speedometer reading on the dash? That is the only thing that seems to make sense to me. The only other explanation is that my Jeep is really smart and realizes she is running on 35's now, so she adjusted her speedo to save me a few bucks.

Does anyone have an answer for this???

 

I upgraded to the exact same tires a few weeks ago as well, and did this exact same thing! My GPS reads the exact same as my speedo. I was very confused...

 

It must be the tires

I am glad that someone else experienced the same thing...It really makes no sense to me. My guess is that the navigation system is integrated to such a degree with the computer than it actually gets it's speed reading from the computer and not via satellite. I want to borrow an after-market navigation system and check the reading on that. I suspect I will find the difference I expected once I do that.

BTW, how do you like your new tires? I think they are a great compromise between a mud tire and an all-terrain tire. The ride is great, but they still have a nice aggressive tread pattern...I love them, and they were the cheapest tires I could find in that size.

 

Download a GPS App onto your smartphone and see what it reads in MPH. My guess is your nav unit us tied into speedo somehow

 

Your speedo is off my brothers did the same thing and his was off and so was mine check with speedo app on your phone or another GPS in a different rig to make sure that the reading you are getting is accurate. Mine was off by about 19% due to the fact that I have the 3.21 gears my brothers wasn't that bad. I think he was about 14% and we both have 2013 JKUs his is the Sahara and mine is a sport. You will need to get an aev procal to correct the speedo. You,can ask tooadvanced as he,can explain it better than I

 

I tried that, but for some reason we couldn't get a good reading...Maybe the app just sucked. I know someone who has a plugandplay navigation system...I will borrow it and see what happens. I am a little disappointed that the factory navigation is tied to the computer...I always thought it was reading my speed via satellite...I used to be impressed by how close the speedo and navigation matched...I am no longer impressed

 

yea the tires have been awesome so far! haven't been on the dirt yet, do to working to much, and waiting for my lift to come still... But on the road im impressed how little noise they make, and how they handle!

 

Check it off a $0.99 app called "speedometer" for iPhone, It will be off for sure.

 

Hahaha don't expect magic or highend systems in a Jeep. We are about 12-15% off from factory with 35s. Haven't reprogrammed yet tho. Been a year and a half now. Most Nav systems are integrated with the computer these days. Few mph readings are from the GPS. Best to use a portable GPS to get the real speed. Don't be disappointed, you just expected too much lol

 

The speed reading on the navigation system is not from the GPS but from the speedometer, that is why the speedo & navigation speed read out is always the same.

I used a Procal to reset for my tires (35" KM2s) and the speedo/navigation unit is stil about 1.5 mph off then on my 'smart' phone app and on my Garmin Montana (both the app & Montana read the same). I also have used the local RADAR displays around town as well to confirm my speed.