Speedometer Calibration
Thread Starter
JK Enthusiast
Joined: Jan 2008
Posts: 383
Likes: 0
From: Lancaster, CA
Speedometer Calibration
I finally got around to putting 35's on my new Jeep and noticed something strange about the speedo. After I left the tire store I wanted to see how far off the speedometer is so I turned on the nav on the radio and noticed that the speed on my nav system and the speedometer were exactly the same. So I'm thinking that the radio gets a speed signal from the computer instead of the internal GPS so I turn on the GPS on my phone, son of a gun, all three devices are showing the exact same speed. At 60 mph indicated on the speedo the nav radio shows 60mph and the GPS on my phone shows 60.6 mph. I had checked the calibration of the speedo with my GPS shortly after I got the Jeep and it was spot on.
My question is where does the Jeep's computer get its speed reference. Does it use an input from the radio's GPS to calibrate itself? Magic?
2015 Wrangler Unlimited Rubicon.
My question is where does the Jeep's computer get its speed reference. Does it use an input from the radio's GPS to calibrate itself? Magic?
2015 Wrangler Unlimited Rubicon.
JK Junkie
Joined: Jun 2013
Posts: 3,697
Likes: 2
From: Grosse Pointe, MI
Speed is calculated via the speed senors at all four wheels. I guess it is not impossible that Jeep somehow wired in the GPS, but that sounds really unlikely. I am curious if the shop has a way to change it for you? They may have a star scan or similar tool. That would be my guess.
Thread Starter
JK Enthusiast
Joined: Jan 2008
Posts: 383
Likes: 0
From: Lancaster, CA
Nope, the shop didn't do anything other than swap the tires. It wouldn't take much of a program to compare the expected speed from the wheel sensors and the GPS speed from the nav and then calculate the new multiplier that is used to calculate speed, but I doubt they would have done that too. Now if I could just find a way to adjust the TPMS alarm set point without buying a Pro-Cal...
JK Newbie
Joined: Feb 2015
Posts: 14
Likes: 0
From: NY
I experienced a similar situation. I bought 33's I don't have an In dash nav so I checked against GPS on my cellphone, and it off less than most people have said on the forum and the online calculators you can find. I only experienced a 0-0.5 mph change opposed to the 2-5mph that I have read about.