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 Post subject: Re: Speedometer calibration and shifting points
PostPosted: Mon Feb 08, 2010 1:43 am 
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so i ended up taking the libby back to the dealership to get the pinion reset adjusted. they said they were off about 20 revs per mile, but now i am off even more. when the speedo says 30 i am really doing 35ish at 60 i am doing 68 ish. wondering if anyone has had this happen, and if so what is the cause. i am wondering if one of the ABS speed sensors could be bad. and if so what one the speedo and trans run off of so i can fix this. or if i am able to just have them reset to factory, or by tire size instead of by revs per mile. any help would be greatly appreciated.

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 Post subject: Re: Speedometer calibration and shifting points
PostPosted: Wed Feb 24, 2010 12:03 am 
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vintageroy13 wrote:
so i ended up taking the libby back to the dealership to get the pinion reset adjusted. they said they were off about 20 revs per mile, but now i am off even more. when the speedo says 30 i am really doing 35ish at 60 i am doing 68 ish. wondering if anyone has had this happen, and if so what is the cause. i am wondering if one of the ABS speed sensors could be bad. and if so what one the speedo and trans run off of so i can fix this. or if i am able to just have them reset to factory, or by tire size instead of by revs per mile. any help would be greatly appreciated.


Glad to see I'm not the only one suffering a similar issue.

I've found that when my speedo is reporting 110 km/h (68 mph) my GPS is reporting 102 km/h (63 mph). I've also driven alongside one of those new Honda Civics with the LCD speedo readout and confirmed what I am seeing is accurate. I spoke with the dealer and they had my Jeep out and reported back that everything was fine but I'm not convinced.

Is there a simple way to fix this???? I was thinking about simply replacing my current 225/75/16 tires with something a bit larger such as a 245/75/16 later this year when I purchase a good set of tires to replace the cheap ones that the dealer put on. Using the tire size calculator at 1010tire.com, it is showing that this size of tire will close the gap and make the speedo pretty close to accurate.

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 Post subject: Re: Speedometer calibration and shifting points
PostPosted: Thu Feb 25, 2010 12:02 am 
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Daksport wrote:
I've found that when my speedo is reporting 110 km/h (68 mph) my GPS is reporting 102 km/h (63 mph). I've also driven alongside one of those new Honda Civics with the LCD speedo readout and confirmed what I am seeing is accurate. I spoke with the dealer and they had my Jeep out and reported back that everything was fine but I'm not convinced.

Is there a simple way to fix this???? I was thinking about simply replacing my current 225/75/16 tires with something a bit larger such as a 245/75/16 later this year when I purchase a good set of tires to replace the cheap ones that the dealer put on. Using the tire size calculator at 1010tire.com, it is showing that this size of tire will close the gap and make the speedo pretty close to accurate.


That is about what my speedo is out as well versus GPS and Honda digital speedos.

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 Post subject: Re: Speedometer calibration and shifting points
PostPosted: Fri Mar 19, 2010 12:38 pm 
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Daksport wrote:
vintageroy13 wrote:
so i ended up taking the libby back to the dealership to get the pinion reset adjusted. they said they were off about 20 revs per mile, but now i am off even more. when the speedo says 30 i am really doing 35ish at 60 i am doing 68 ish. wondering if anyone has had this happen, and if so what is the cause. i am wondering if one of the ABS speed sensors could be bad. and if so what one the speedo and trans run off of so i can fix this. or if i am able to just have them reset to factory, or by tire size instead of by revs per mile. any help would be greatly appreciated.


Glad to see I'm not the only one suffering a similar issue.

I've found that when my speedo is reporting 110 km/h (68 mph) my GPS is reporting 102 km/h (63 mph). I've also driven alongside one of those new Honda Civics with the LCD speedo readout and confirmed what I am seeing is accurate. I spoke with the dealer and they had my Jeep out and reported back that everything was fine but I'm not convinced.

Is there a simple way to fix this???? I was thinking about simply replacing my current 225/75/16 tires with something a bit larger such as a 245/75/16 later this year when I purchase a good set of tires to replace the cheap ones that the dealer put on. Using the tire size calculator at 1010tire.com, it is showing that this size of tire will close the gap and make the speedo pretty close to accurate.


This is exactly the same problem I noticed driving on the 403 the other day,I currently have the stock Wranglers in 225/75r16 I figured on moving up a size in the near future to replace my worn wranglers and figured this would be an easy enough way of getting things more in-line as well.


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 Post subject: Re: Speedometer calibration and shifting points
PostPosted: Wed Jan 18, 2012 4:27 pm 
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Talked to Keith at GDE, he told me to reverse calculate the rpm's. So we (Dealer)did that and ended up programing it to 765 rpm's for the 265 75 16 and it is now within .5 MPH, but the odometer is off approx 9%.

Lanny at Tony Martins Jeep in Platte City, Mo. 816 858 5575 cost $50.00

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 Post subject: Re: Speedometer calibration and shifting points
PostPosted: Fri Aug 11, 2017 12:05 am 
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Back from the dead. The revolutions per mile is either a geometric calculation assuming ideal diameter, no slip between the road, and a straight line, or that calculation with a fudge factor for slip and turns, or some empirical measurement. If you assume ideal geometry, and you take the reciprocal of the rev per mile, multiply by (5280*12/pi) = 20168, then you get the diameter of the tire in inches. Similarly if you divide 20168 by the diameter of the tire in inches, you get the revolutions per mile. A 32" tire, should have 630 rev per mile. Tire rack lists the Goodyear Duratrac 235-85-16 as 31.7" diameter and 656 rev per mile. Based on ideal geometry and straight line rolling, it should be 636 rev per mile, so there is something else in the calculation. Maybe something to do with the contact patch or that the tire is not a circle. Another example is the 32 x 11.50 x 15 Goodyear MTR. Tire rack lists the diameter at 31.8 and rev per mile as 654. Based on 20168/31.8 the rev/mile should be 634. Both examples deviate by 20 rev/mile.

So, I took a look at two different Goodyear tires, Wrangler HT and Duratrac. I downloaded all of the tire diameters and Rev/Mile for all of the sizes available into excel, and plotted. Then fit a line, and all of the data fell very close to the equation Rev/Mile = 1303.9 - 20.35 * (diameter in inches). The r-squared value for this fit was 0.9941 which means it follows this equation very well. I didn't bother to look at other tire manufacturers, and this entire exercise is pretty pointless since tirerack.com has this info for almost every tire, but if you can't find the rev/mile data for your tire, and you know the diameter, this equation should get you very close to the right value.

Also, some scanners can change tire size. I used a friend's Autel scanner to change the tire size without going to the dealer. It allows access to the body control module.

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 Post subject: Re: Speedometer calibration and shifting points
PostPosted: Fri Aug 11, 2017 4:14 am 
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Dent wrote:
Back from the dead. The revolutions per mile is either a geometric calculation assuming ideal diameter, no slip between the road, and a straight line, or that calculation with a fudge factor for slip and turns, or some empirical measurement. If you assume ideal geometry, and you take the reciprocal of the rev per mile, multiply by (5280*12/pi) = 20168, then you get the diameter of the tire in inches. Similarly if you divide 20168 by the diameter of the tire in inches, you get the revolutions per mile. A 32" tire, should have 630 rev per mile. Tire rack lists the Goodyear Duratrac 235-85-16 as 31.7" diameter and 656 rev per mile. Based on ideal geometry and straight line rolling, it should be 636 rev per mile, so there is something else in the calculation. Maybe something to do with the contact patch or that the tire is not a circle. Another example is the 32 x 11.50 x 15 Goodyear MTR. Tire rack lists the diameter at 31.8 and rev per mile as 654. Based on 20168/31.8 the rev/mile should be 634. Both examples deviate by 20 rev/mile.

What you are encountering, and what they are likely figuring for is the difference between the mounted unloaded (off the ground, fully round) circumference, and the circumference calculated by what's known as "Effective Rolling Radius".
Unless you have steel tires like a train, there is always some deflection that results in what you refer to as "slip"
They figure the rev/mile from the tire mounted on a proper spec wheel, and inflated correctly, with a load properly within it's capacity.
You start with the mounted unloaded tire circumference.
Them take the loaded tire flat deflection (say, a 10" long patch) and you subtract that 10" from the length of the same 10" drawn point to point through the arc of the unloaded radius (will be slightly longer) so you have the difference.
Then subtract that difference from the total mounted unloaded circumference.
There you have the Effective Rolling Circumference.
That's what you use to figure the revolutions / mile.... :dizzy:

As smaller tires tend to operate closer to the limits of their weight ratings, because of the nature of arced lines vs straight lines, other factors being equal, the smaller the tire diameter, the more % difference there will be between the mounted unloaded circumference revolutions/mile, and the actual Effective Rolling Circumference rev/mile :5SHOTS:

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 Post subject: Re: Speedometer calibration and shifting points
PostPosted: Sat Aug 12, 2017 12:23 am 
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GordnadoCRD wrote:

They figure the rev/mile from the tire mounted on a proper spec wheel, and inflated correctly, with a load properly within it's capacity.
You start with the mounted unloaded tire circumference.
Them take the loaded tire flat deflection (say, a 10" long patch) and you subtract that 10" from the length of the same 10" drawn point to point through the arc of the unloaded radius (will be slightly longer) so you have the difference.
Then subtract that difference from the total mounted unloaded circumference.
There you have the Effective Rolling Circumference.
That's what you use to figure the revolutions / mile.... :dizzy:

As smaller tires tend to operate closer to the limits of their weight ratings, because of the nature of arced lines vs straight lines, other factors being equal, the smaller the tire diameter, the more % difference there will be between the mounted unloaded circumference revolutions/mile, and the actual Effective Rolling Circumference rev/mile :5SHOTS:


If that is the procedure, then they are using the same procedure to determine the diameter of the tire in the spec data and rounding the numbers to the nearest whole number, at least that would be my assertion since the correlation between reported tire radius and rev/mile is such a good linear fit and given one spec, diameter or rev/mile, one could determine the other. Seems odd that there are a few tires without a reported rev/mile.

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