Turn signal issues and tire size reset?

[Delhux]

So I am on about a 300-mile trip today—starte diff in the rain, all highway.

I come to an intersection and signal a left turn, the dashboard signal indicator starts to blink as usual, but then it just gets stuck on—solid green arrow. I toggle the turn signal and it goes back to working.

Weird, but I didn’t think much of it.

A few miles later a signal a right turn onto an on-ramp. This time, as I’m traveling down the on ramp (about when I would expect the right turn signal to turn off) I notice that both indicators are blinking as though I pressed the hazard button (I definitely did not). I toggle the hazard button and it goes back to normal.

A few miles later I notice that my GPS/speedo seem out of sync. Sometimes the GPS is just wrong, so I restart the app and give it some time, but it stays inaccurate.

I notice that at 70 on the dash, it’s reporting 75mph on the GPS.

So I start checking the distance between mile markers on the dash—it seems to be off by about 1/10, so I knew it going for a 20-mile test—at the 20-mile mark, my dash reports 18.6-miles.

I have 35” tires.

I am stopped for a rest break now, the rain has mostly stopped, and I am hoping it starts back up with working turn signals and correct tire diameter. I am just surprised that it seemed to “default” back to smaller tires mid-drive, seemingly after the turn signal issues.

I assume rain must be a factor, but I didn’t hit any big puddles—just highway rain.

Thoughts?

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[yoda13]

That’s very weird. Sounds like an electrical gremlin. Hopefully, stopping for a bit clears up the problem. I haven’t heard of this. I hope it doesn’t continue to be an issue.

 

[Mac]

Did you use something to adjust for larger tires prior to this?

 

[TheGreatCO]

35" tires are about 6" smaller in circumference than 33" tires. Works out to about 6% larger, but that really depends on the true diameter with inflation. Over-inflating could make them larger.

If I check the 20 miles vs 18.6 miles, that's about 7% error, which jives with the 33" vs 35" difference of ~6%.

If i check the 70 mph vs 75 mph, that is also about 7% off, which again jives with the 33" vs 35" difference of ~6%.

All of this sounds like the car thinks it has smaller tires than it does.

 

[Delhux]

Did you use something to adjust for larger tires prior to this?

I had the tires put on by Tri-City when I ordered it, they must have done something to ensure the speedo is accurate.

 

[Delhux]

35" tires are about 6" smaller in circumference than 33" tires. Works out to about 6% larger, but that really depends on the true diameter with inflation. Over-inflating could make them larger.

If I check the 20 miles vs 18.6 miles, that's about 7% error, which jives with the 33" vs 35" difference of ~6%.

If i check the 70 mph vs 75 mph, that is also about 7% off, which again jives with the 33" vs 35" difference of ~6%.

All of this sounds like the car thinks it has smaller tires than it does.

So I’m on my second test break now.

The first stop, I left the car off for about 5-minutes, when I started again the GPS and speedo were back in perfect sync.

However, I repeated the mile marker test and found that in a 10-mile test, the trip odometer reported 9.3 miles—even though Speedo and gps were lined up this time.

I’ve never before questioned the accuracy of the odometer, since the speedo/GPS test has always lined up.

Is it even possible to have the car recognize the correct size for the speedo, but then record mileage based on a different value?

 

[TheGreatCO]

So I’m on my second test break now.

The first stop, I left the car off for about 5-minutes, when I started again the GPS and speedo were back in perfect sync.

However, I repeated the mile marker test and found that in a 10-mile test, the trip odometer reported 9.3 miles—even though Speedo and gps were lined up this time.

I’ve never before questioned the accuracy of the odometer, since the speedo/GPS test has always lined up.

Is it even possible to have the car recognize the correct size for the speedo, but then record mileage based on a different value?

If you put larger tires on the car, you need to tell the car. Some scanners can do it, or ask the dealer. The odometer is just a counter, counts how many times the transmission spins, multiplies it by gear ratios and tire sizes to get distance traveled. If any of those multipliers are wrong, the speedo and odometer will be wrong.

FWIW, the speedo and odometer are always slightly inaccurate, due to the nature of how they work. As tires wear they change size, tire pressure affects the size, ambient temperature and operating temperature change the tire size, speed changes the tire size, all of which affects distance traveled per tire rotation.

 

[Delhux]

If you put larger tires on the car, you need to tell the car. Some scanners can do it, or ask the dealer. The odometer is just a counter, counts how many times the transmission spins, multiplies it by gear ratios and tire sizes to get distance traveled. If any of those multipliers are wrong, the speedo and odometer will be wrong.

FWIW, the speedo and odometer are always slightly inaccurate, due to the nature of how they work. As tires wear they change size, tire pressure affects the size, ambient temperature and operating temperature change the tire size, speed changes the tire size, all of which affects distance traveled per tire rotation.

For sure.

I finished my trip and the speedometer and odometer continued to be out of sync.

Speedometer is now accurate.

Odometer is still off by approximately 7%.

Nothing about tire wear would account for that.

I’m just having a hard time understanding how the vehicle’s computer could or would be calculating the revolutions for the speedometer differently than the revolutions for the odometer—it seems very odd the computer wouldn’t just have a single variable for tire size that dictates both speed and distance calculations, rather than (apparently) having distinct variables for both.

It just strikes me as very weird.

 

[Delhux]

Also, I was looking at the JSCAN tool—it clearly states it can adjust tire size to resolve speedometer conflicts, but doesn’t seem to clearly state if it resolves odometer inaccuracies.

Does anyone know for sure if JSCAN corrects odometer readings when updating tire height/diameter?

 

[Renegade]

Do you have your phone connected to the Jeep when using your speed app? Some apps pull the speed from your Jeep speedometer when connected. If you were connected the second time but not the first, that would explain the variance.

 

[Delhux]

Do you have your phone connected to the Jeep when using your speed app? Some apps pull the speed from your Jeep speedometer when connected. If you were connected the second time but not the first, that would explain the variance.

That’s interesting. I was using Waze connected via CarPlay on my iPhone, but was plugged in each time. I’ll have to try it without plugging in to see if it is wrong…

 

[md_adirondacks]

That’s interesting. I was using Waze connected via CarPlay on my iPhone, but was plugged in each time. I’ll have to try it without plugging in to see if it is wrong…

Waze 100% pulls speed data from the car computer, unless it malfunctions or its connection isn't working. Try using it while the phone is not plugged in, that way you'll see your speed as read by GPS. That's actually a good way of setting it yourself if you have a Tazer or something like it.

I'm pretty convinced that your install shop never changed your wheel size in the computer and what you're seeing is your phone connection being inconsistent. Which plug are you using btw, the one in the front or inside the arm rest? The plug inside the arm rest is known for not maintaining a good connection and in mine never works at all past 5 minutes.

 

[Delhux]

Waze 100% pulls speed data from the car computer, unless it malfunctions or its connection isn't working. Try using it while the phone is not plugged in, that way you'll see your speed as read by GPS. That's actually a good way of setting it yourself if you have a Tazer or something like it.

I'm pretty convinced that your install shop never changed your wheel size in the computer and what you're seeing is your phone connection being inconsistent. Which plug are you using btw, the one in the front or inside the arm rest? The plug inside the arm rest is known for not maintaining a good connection and in mine never works at all past 5 minutes.

Yup, arm rest here.

I’m willing to bet Tri-City either never reprogrammed for the tires or when another dealership replaced my battery sensor, or when another hacked my Mopar lift and then tried to hard sell my a tazer…

I think I’ll pick up the adapters for JSCAN and just answer the question for myself. Honestly, anything to avoid scummy Jeep dealerships at this point…

 

[Delhux]

Update:

I hooked up the JSCan tonight and was a little surprised by the tire value that the system had before I changed anything:

I’m pretty sure the OE tires were 32”, which Tri-City upgraded to 35”, but the tire diameter was set to 32.74”.

I know a lot of people say to set 35’s to 33.75” and 37’s to 35.5”, etc, but I’m not making sense of 32.74” for either 32” or 35” tires.

Weird…

In any case, Ove reset the tire diameter via JSCAN to 34” and did a quick test drive at 45MPH and found the vehicle speedometer was still reporting slower than the GPS speedometer, but now it was onlyoff by approximately 1MPH @ 45MPH.
I’m going to leave it as-is for a few days and then decide if I want to bump it up by a quarter inch to see if I can get it accurate. I will also need to do another mile-marker test to verify.

 

[Mr._Bill]

Did you measure the actual size of the tires? Go from the center of the hub to the ground and then double it. That should provide a starting point that is real close to what is needed. Then adjust to get it accurate.

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