Incorrect speed?

Boothorp

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I have had issues with my speedometer being a little off. I have been on the highway and locally where there are speed signs up multiple times, and they do not match my speed by 3-7 mph. I have 35x12.5 tires and am wondering what’s going on?

I have the newest update.

 

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Your tires are larger than the stock tires. A larger diameter has a longer circumference (Pi times the diameter), which causes the tire to travel further for every revolution of the axle. The speedometer displays the speed based on the number of revolutions of the tire. Since the larger diameter tire covers more ground in 1 revolution, the axle rotates less times every mile. In doing so the Jeep reads a lower speed.
This discrepancy is more pronounced the faster you go.
 

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@Boothorp

Not sure if you have a smart phone but when programming it really is a good idea to program and then using a smart phone app that gives you a mile per hour (MPH) readout, go/drive and check to see that your speedometer is accurate.

I would not only verify speedo accuracy with those "speed signs".

It is a good idea to verify with the app at varying speeds, such as 30, 50, 60, 80.

IF, you find your speedo off fool with your programming in regard to tire size until your speedo is correct a varying speeds.

The app I use, is GPS PLUS.

.
 

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To do some math for you:
The stock tire on the Sahara is 32". That means it travels 100.5" per revolution, or roughly 633 times per mile.
The larger tire being 35" will travel 109.9" per revolution, or roughly 576 times per mile.
The difference between the tires is roughly 10% (both diameter and circumference). As a result the tire will rotate roughly 10% less times for every mile traveled. As a result your speedometer will display a speed roughly 10% slower than your actual speed.
Therefore at 35 mph your speedometer will show 3.5 mph slower, resulting in a display of 31-32.
As you go faster it's more pronounced. At 70 mph your speedometer will show 7 mph slower, resulting in a display of 63 mph.
 
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Boothorp

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I apologize. I understand the math. I guess posting in the Superchips forum I implied that I have already calibrated and still may be incorrect.
 


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Boothorp

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I’ll download the app and check it out.
 
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This was at 80 mph. I recalibrated, updated and recalibrated again. Is it my tires are not actually 35x12.5?

5C2D5DD4-1B8C-4B74-BC8F-1753A162BCE2.png
 

WranglerMan

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If you are running larger than stock your speedo will be off and so will your odometer so you will either have to have the dealer flash your system for larger tires or go the route of a programmer of your choice and there are several choices ( Tazer JL, Tazer Lite and Flashcal ) I personally have the Tazer JL and it’s a pretty simple adjustment for tire size but you will have to DL a speed app to your smartphone, mine was off 5 mph when I went to 315’s but now it’s exactly perfect from 75 mph and down

The other choice is do nothing and just remember your speedo is off, the shift point if you have an AT will adjust after driving it for awhile but won’t be the same as stock due to the additional weight and rolling mass of the tires plus you will drop some mpg’s But hey it’s a Jeep and not a Prius
 

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Will, he's using a Superchips to reprogram/adjust it.

Josh, it sounds like the issue then is that your tires aren't the same as what you've adjusted for. Not to worry, they rarely are exactly the specified size. They should be close though. The easiest thing to do is go to the manufacturer's website and see the actual size of the tire. They'll have them spelled out in detail including width on a given rim. The diameter is mostly unaffected by the mounted rim width thanks to steel belts, but the width does change so they specify it. What tires are you running?
 

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I apologize. I understand the math. I guess posting in the Superchips forum I implied that I have already calibrated and still may be incorrect.
Nah, my mistake. I assumed you were trying to figure out why you had to reprogram it. Makes sense now that you explained it. Oops. :facepalm:
 


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In running Nitto Ridge Grapplers 35x12.5 R 18
 

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Great choice. I considered those once, but they weren't released yet. I got some MIckey Thompsons instead because I didn't want to wait for them.

That specific tire, per Nitto, is 34.76" diameter. That's 0.7% smaller than you have it programmed. So using the note you made with the screen shot, that your speedometer showed 80 when that data was captured, the gps should have shown 79.4 mph based on how you've got the Superchips programmed. Keep in mind that GPS may not be exact, and the speedometer of most cars/trucks aren't certified to be exact either.
If you captured the above image after slowing slightly from 80 you may have a pretty close reading after all. The max speed shows 78.4, which is pretty close considering the accuracy of the uncertified speedometer and of gps. 2 mph is considered accurate, and even police radar is only accepted to be accurate with 2 mph. If that's all the further off it is at 80 mph, I'd go with it.
 

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I have had issues with my speedometer being a little off. I have been on the highway and locally where there are speed signs up multiple times, and they do not match my speed by 3-7 mph. I have 35x12.5 tires and am wondering what’s going on?

I have the newest update.
I have Nitto Ridge Grapplers 35 x 12.5 x 17 with 34.76 external measurement. At 28 PSI, changing the tire size to 34.25 gets most accurate results.
 

WranglerMan

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I to had issues with my speedo being off and I’m running a Tazer JL, I am rolling on 315x70xR17’s at 30# and the tires measure right at 34” with a straight edge laying across the top but I was off right at 5 mph using a GPS speed app so I just kept bumping up the size in the programmer and testing and finally ended up at 34.49” before I was spot on at 75 mph and down so I would go up or down and keep checking with the speed app.
 

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Has anyone tested the accuracy of the speed app against the speedometer with stock tires?

 

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