Speedometer learning??

[Viper501]

2022 Mojave. When I bought it I had the stock metric 33’s on stock wheels. I did an AEV 2 inch lift and recently upgraded to AEV Borahs with Mopar TPMS and 315/70/17’s. I did not use the AEV module to update the tire size.

Originally I saw a 4 mph difference between a speedometer indicated 70 and the Waze showing GPS at 74. I planned to use the AEV or jscan or tazer to set the tire size to 34.4 ish.

However I’m on a relatively long trip and now there is no discrepancy between the speedometer and the waze GPS. Has anyone ever seen the speedometer somehow auto correct? I’ve got the 8.4 and CarPlay running on it. I just don’t understand how this is going on. And I’m sure I should have searched first but I didn’t so apologies in advance.

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[Jefe1018]

I would double check with multiple apps, if on iOS I like to use the navigation based and one called “Speed Box”.

 

[kevman65]

If the device you are running Waze on is connected either through blue tooth or through a USB cable to the JT, disconnect it completely from the JT and try running it again.

There have been instances where when connected the device mirrors the speedo but when completely disconnected gives a different value.

 

[Kevin_D]

And you'll get better results using the odometer and checking distance over several miles, rather than the speedometer.

Kevin

 

[Viper501]

So I was skeptical of this being a possibility since I’d stopped several times along the way, sometimes for over an hour. Each time I’d unplugged the phone and taken it with me. In fact after posting this question I got back in the truck and got back on the road. I started the destination over and continued to get synchronous readings from the Waze on CarPlay and the speedometer. As I was traveling I thought I was passing a lot of cars and trucks. Way more than the usual dawdlers.

Out of curiosity I tried unplugging from CarPlay and initiating a new route guidance session on Waze. Magically the correct speed reappeared and the differential was back.

Apparently the theory is correct. Somehow Waze is taking the speedometer input and applying it to the display through CarPlay. It will correct itself if you unplug it then plug it back into CarPlay. I’m wondering if it does that when there is a problem with GPS reception.

So it seems I’m back to Re programming or doing my 4 mph subtracting.

 

[ShadowsPapa]

The bottom line on these is that the speedometer doesn't learn or self-adjust. It relies on the tire diameter (actually it's the circumference in the BCM in MM) on determining speed and distance.
I set my trip meter to 0, I use a GPS NOT connected to the truck and drive 15-20 miles and then compare the trip meter to the GPS distance reading and go from there.
Speedometers are off by a percentage so if it's off 4 mph at 70 it would be off by less at 30 and by more at 90, for example.
I prefer distance traveled when calibrating.

 

[Viper501]

The bottom line on these is that the speedometer doesn't learn or self-adjust. It relies on the tire diameter (actually it's the circumference in the BCM in MM) on determining speed and distance.

…..
Speedometers are off by a percentage so if it's off 4 mph at 70 it would be off by less at 30 and by more at 90, for example.
I prefer distance traveled when calibrating.

Agreed on both counts. Given the technology of the modern cars it *could* be instructed to learn from the additional inputs of the gps signal both from an external source as well as the built in navigation system. Obviously that’s not the case for Jeep.

And the percentage of error definitely increases in proportion to the speed for obvious reasons.

 

[RMFSJT]

Man I was wondering this same thing on my 23 this explains it perfectly. Although I did go in and update my tire size with Jscan, before that I thought I was going crazy when Waze and my speedometer were matching.

 

[Jason Oliver]

Agreed on both counts. Given the technology of the modern cars it *could* be instructed to learn from the additional inputs of the gps signal both from an external source as well as the built in navigation system. Obviously that’s not the case for Jeep.

And the percentage of error definitely increases in proportion to the speed for obvious reasons.

No, it does not. The percentage remains the same. The mph error increases with speed for obvious reasons.

 

[Almost]

When using your phone for GPS i believe the reason it’s taking your vehicle speed into account is so it can update when you loose service through tunnels for example.

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