Self Correcting Speedometer

[JeepPunk]

I’m almost always using CarPlay so?? Lol who knows. I’ll try without tomorrow.

Found a member from a previous thread about this issue and that’s what is happening.
When iPhone plugged into Jeep;

GPS even simple speed apps do 100% match what Jeeps speedometer says.
However and unfortunately it is not correcting speedometer. Instead and who knows why the Jeep is sending data to phone and changing speed readings.
The reality is just about as bonkers as what I had initially hoped for just not as useful.

I even tested it today; Set cruise in 5th gear at 55 while hooked to CarPlay with speedometer ap opened. Disconnected lightning cable and app immediately went from 55 to 58!
Wonder if this is just an iOS peculiarity or of android based devices do this too.
Anyhow mystery solved: I still think my Jeep is magical though.

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[WranglerMan]

When I set my tire size to correct my speedo with my Tazer I used a stand alone app not connected and after a few changes I was able to get it spot on from 70 mph and down, I have not heard the speedo fixes itself bu have heard that the tranny does readjust shift points after tire size has been corrected, I do find that I jump between 7-8 since going to 315’s with the lift but I was told that’s pretty normal since I added the tires and lift, has something to do with rolling mass

 

[WRANBO]

Funnily enough, my speedometer reads accurate as well. I also have the 8.4 nav unit, I wonder if the speedo is calculated through GPS?

 

[Dyolfknip74]

Funnily enough, my speedometer reads accurate as well. I also have the 8.4 nav unit, I wonder if the speedo is calculated through GPS?

No, it isn't. There are literally 1000s before you, myself included, who have calibrated for larger tires. You didn't just get a unique Jeep. I promise.

 

[WRANBO]

No, it isn't. There are literally 1000s before you, myself included, who have calibrated for larger tires. You didn't just get a unique Jeep. I promise.

Third party GPS apps say otherwise. They were in sync with my speedo. Odd for sure.

 

[Dyolfknip74]

Third party GPS apps say otherwise. They were in sync with my speedo. Odd for sure.

What size tires did you have did you to? What gearing?
Did you swap wheels as well? What is weight difference between stock wheels and tires and new ones?

 

[WRANBO]

What size tires did you have did you to? What gearing?
Did you swap wheels as well? What is weight difference between stock wheels and tires and new ones?

Tire: 245-75-17 to 285-70-17
Rims: same
Gearing: 3:45 stock
Weight difference: 9 lbs > stock per wheel

 

[Dyolfknip74]

Tire: 245-75-17 to 285-70-17
Rims: same
Gearing: 3:45 stock
Weight difference: 9 lbs > stock per wheel

Not too big a change I guess but I'm still sceptical lol.

Anyway, enjoy your rig. Cheers

 

[Strommen95]

Tire: 245-75-17 to 285-70-17
Rims: same
Gearing: 3:45 stock
Weight difference: 9 lbs > stock per wheel

The factory speedo is actually off, it reads a bit fast with 245/75/17 tires. 285/70/17 is either just right or slightly off at higher speeds, but in my experience, more accurate than stock with 245s.

 

[WRANBO]

Not too big a change I guess but I'm still sceptical lol.

Anyway, enjoy your rig. Cheers

Not too much, no, but still within measurable percentage differences for sure. It’s an interesting thing, lol. I just came across this thread and never took notice until I got here. Thanks, you too!

 

[WRANBO]

The factory speedo is actually off, it reads a bit fast with 245/75/17 tires. 285/70/17 is either just right or slightly off at higher speeds, but in my experience, more accurate than stock with 245s.

Interesting! I never thought to GPS check the speedo with the 245s. This is a funny thing for sure though! Who knew that buying tires differently sized from factory would produce a more accurate speedo?

 

[OldBlue]

The factory speedo is actually off, it reads a bit fast with 245/75/17 tires. 285/70/17 is either just right or slightly off at higher speeds, but in my experience, more accurate than stock with 245s.

I run Waze on my drives back and forth to work, and noticed the same thing. Mine (w/ 245/75R17) is off 2-3 mph.

 

[W5MQS]

Well my Jeep is pretty magical lol. I’m super skeptical of this myself; however I’ve used this exact same method to calculate speedo error in a pickup after going up in size and my predictions based on manufacture actual diameters from tire rack where very close to what my speedometer app showed.

Factory Rubi (33”) are listed as 32.8” and my new ones are 34.6”. And while there may be some difference based on my wheel diameter and what air pressure I’m at it should be close.
So my actual speed should be approx. 5.5% higher than what Jeep shows. At 65 I should actually be going about 68.
BUT at 65 Jeep speedo my ap actually read 64? So maybe there was high solar flare activity yesterday, the gps satellite was hacked by Russians, or maybe it was just a dream.
It was weird though and thats why I searched the forum yesterday.
Defineatly will be looking into it further because I’d rather not get a ticket “thinking” it was right.

Negative, Ghost Rider...that was not caused by a solar flare or the Ruskies but was most likely due to the rotation of the earth, global warming and el Nino'...if you don't believe that...it was probably some facsimile thereof related to the square root of Pi divided by the product of the inverse square law x2. Otherwise, it's probably waaaay more complicated than that. Just sayin'.

 

[JayJay]

The factory speedo is actually off, it reads a bit fast with 245/75/17 tires. 285/70/17 is either just right or slightly off at higher speeds, but in my experience, more accurate than stock with 245s.

I see the same thing with the stock 285/70/17 tires. Set cruise control on 60 - speedometer reads 62, GPS reads 60.

 

[jeepoch]

Another way you can check your tire size calibration accuracy is with the odometer. With a larger diameter tire means a higher tire circumference. This will ultimately yield a longer distance traveled per wheel rotation. So with a larger tire than that delivered as stock from the factory (assuming the powertrain engineers calibrated everything properly), your vehicle will then require fewer wheel revolutions as measured over any unit of distance.

Therefore you can check your odometer on the freeway by checking it against the standard milepost markers. Larger tires will traverse a mile (distance between the mileposts) with a lower number of wheel rotations. So if your tire size (diameter) is set for a smaller tire, your odometer will show less than a mile between markers.

This also directly translates into having a lower 'indicated' speed (as shown on the speedometer) than the vehicle's 'true' velocity. In addition, this speed delta will be exactly proportional to the percentage difference between the diameter of the stock and larger tires.

So with a larger tire size than calibrated means you'll have a higher propensity to collect speeding tickets, especially at highway speeds where the difference error will be greatest. This overall error value will clearly increase as speed increases by that specific percentage amount.

For an example; moving from a 31 to a 35 inch tire (31/35 = 0.886) which yields a 11.4% tire size difference. At 25 mph 'indicated' would actually be 27.85 mph 'true' (almost a 3 mph difference) no big deal. But at 75 mph 'indicated', would be 83.55 mph 'true' (almost a 9 mph error). So if you normally drive just less than 10 mph over the speed limit (based on your speedometer), you are really traveling nearly 95 mph. Good luck avoiding tickets!

On the bright side however, if you normally don't drive anywhere near the speed limit, you would certainly enjoy a longer warranty period since it will appear that you are driving 11.4% less distance than reality. If you installed these 35's when your Jeep is brand new you would actually get just over 40K miles on your standard 36K warranty. Again, just 0.884 miles on the odometer for every real mile traveled.

For reference, I've personally opted for accuracy. Using my Tazer, I've set my tire size calibration value to 34.8" for my Goodyear Duratrac All-Terrain Wranglers (35x12.5x17).

Have fun,
Jay

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