# Calibrating Speed on Speedometer

#### Taken1983

Joined Nov 5, 2017
17
Hello can someone please help me I am trying to recalibrate the speed on a 2014 Jeep Wrangler. The situation is the needle is lower then the vehicle is actually going for example the needle is at 80km per hour the Jeep is actually going 100km per our. The needle is at the 0 kph when the vehicle is stopped but the faster you go the more the speedometer is out. What I am looking for is to program the eeprom chip so that the needle sweeps properly. Does anyone know the line location on the eeprom chip where I change the hex coding. Please feel free to pm as well.

Thank You,

#### RichardO

Joined May 4, 2013
2,271
Unless you have changed to really large tires I think you may have some kind of failure in the speedometer.

Does the odometer register the right distance?

#### joeyd999

Joined Jun 6, 2011
4,429

#### bwilliams60

Joined Nov 18, 2012
1,343
Is this the original instrument cluster from the vehicle?
Has it been changed from miles to kilometers or vice versa?
Has anyone changed the wheels on this vehicle?
Has any programming been done to it?
Has there been any aftermarket speed sensors or tone rings installed on the unit?
Something had to change to make it read differently. Coding in the EEPROM will not change by itself.

#### Tonyr1084

Joined Sep 24, 2015
5,067
My 08 Tacoma (Taco) had a similar issue. At 65 MPH indicated speed my actual speed was 58 MPH. Part of the issue was the tire was 3% undersized, accounting for a small amount of the error. The rest was due to a 29 tooth gear on the speed sensor. When I changed to a 30 tooth gear on the sensor it brought my speedometer and odometer into near spot on. But I had to move my speedometer needle to "Two miles per hour BELOW Zero." When driving at 25 MPH Indicated (MPHI) my actual speed was 26 MPH. At 50 MPHI my speed was spot on. At 75 MPHI my actual speed was 74 MPH. So I was pretty happy with that.

As tires wear your MPHI and actual speed change. Worn tires and you're going slower than indicated. If you put oversized tires on then your MPHI will be lower than your actual speed. There was a lot of calculating that needed to be done. Number of rotations of the tire per mile, number of rotations of the drive shaft per one rotation of the tire, and the number of pulses per rotation of the driveshaft. To determine the last with accuracy I had to rotate the driveshaft ten times in order to get a fairly accurate estimate of the ratio of pulses per revolution of the tire. Then divide that by the number of teeth on the sensor drive gear. Then calculate how the next size gear would alter the reading. The original gear had 29 teeth. Switching to 30 teeth made up the 9% I needed. That was in my Taco.

Other vehicles (not Toyota) you can buy a programmer that can tell the computer what size tires you're running and the computer will make the necessary changes to the indicated speed in relation to the size of tire you're running. I don't know this for sure, but I bet most tire shops can flash your computer to reflect proper speed and mileage for the given tire size you choose to run.

#### Tonyr1084

Joined Sep 24, 2015
5,067
Do you know how to calculate the size of your tire? I'm talking height and circumference.

Example: 255/75R18. that's an 18 inch rim in my example.

255 is the width of the tire in millimeters.
75 is sidewall height in percentage of the tire width
R (is for Radial, not all tires are R)
18 is the diameter of the rim.

Multiply 255 by 0.75 (that's the decimal equivalent of 75%), then multiply by 2 (because there is sidewall above and below the rim). Then divide by 25.4 (conversion from MM to Inches). Then add the rim size for the overall diameter of the tire. Finally, multiply that by 3.14 for the circumference. Now you know how far the tire rolls in one revolution.

255 x 0.75 x 2 ÷ 25.4 + 18 = 32.06 inches tall.
32.06 x 3.14 = 100.67 inches forward per revolution.

There are 5280 feet in one mile. That's 63,360 inches per mile. Divide that by 100.67 and you get the number of revolutions per mile. In this case, 63,360 ÷ 100.67 = 629.38 revolutions per mile. Multiply that by the gear ratio of your ring & pinion. In my case, it was 4.1 to 1. My drive shaft turned 2580 revolutions per mile. Keep in mind I'm using theoretical numbers. In high gear (not overdrive) the engine should be turning at 2580 RPM at 60 MPH (one mile per minute). If my original gearing in the speed sensor was 3.8 to 1 then my speedometer should be seeing 9,805.8 pulses per minute per mile (at 60 MPH). Changing from 29 tooth to 30 tooth gearing (calculate 9805.8 ÷ 29 x 30 = 10,143.8) changing from 9805 to 10143 pulses per mile, causing the speedometer to see a higher rate of speed, which matched the true speed to the indicated speed. The last thing I had to do was move the needle on my speedometer. Not difficult to do but in my case you DO have to take the gauge cluster out of the vehicle and remove the clear plastic cover in order to move the speedometer needle.

Oddly enough, this kind of calculation I can do. But calculate an RC circuit? or an RI circuit? Or an RIC circuit? I'm as lost as a dime in the sand at the beach.

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#### bwilliams60

Joined Nov 18, 2012
1,343
Sounds like a lot of math. If you go to the tire manufacturer and look up the tire, they will give you the revs per mile or pulses per mile of the tire.
Speedometers in newer vehicles are all electronic and not to be played with by do-it-yourselfers. They are programmed with all the information they need to be accurate and they compensate for tire wear. If it is not reading properly, it probably needs to be programmed. I think the programming on the OP's Jeep has already been messed with and now he is trying to correct it. There is more to the story which I am waiting to hear.

#### Tonyr1084

Joined Sep 24, 2015
5,067
@bwilliams60 I agree; there's more. And I also agree that the speedometer has already been messed with. Someone put different size tires on the vehicle and re-flashed the computer for the larger tires (or smaller, we still don't know yet). The right way to calibrate the speedometer is to have the computer flashed to the correct tire size that is on the vehicle. Tire shops are my bet on where you can get that done. If not, here in the US there are places that are legally bonded for speedometer work. Flashing the mileage and setting up the vehicle for the correct speed / miles per tire size. In my case, Toyota has been so secretive about how their computers work that almost nobody can re-flash the computer. I think it's "Bully-dog" who has broken the Toyota code and can now flash the computer. I can buy a device for flashing my computer; change tire size, change spark timing, fuel ratio and so on. But the device is over $800 (US). The gear and a little math was a lot cheaper. I don't remember off hand what I spent on the gear for the speed sensor, but I don't think it was more than$20 (US). If more, it wasn't much. It's just a metal shaft and a plastic gear.

On the gauge cluster there's a memory chip that tracks the mileage. There IS a way to re-flash it, but it's not intended for the common vehicle owner to be changing the mileage on the chip. Legal issues and all. With my 08 Taco, there were more miles showing on the speedo than were actually on the vehicle. At least that's the way it was when I bought it.