Speedometer Accuracy

[WranglerMan]

I’m a bit confused but thats normal for me somtimes.

Anyway my speedometer has be set for 35” KO2’s using JSCAN like almost a year back and was spot on but now it seems my speedometer is showing a slightly faster speed that GPS shows, not by much but a small difference.

This a.m on my way home on flat road with my cruise set at 65mph my GPS app jumped between 63-64 mph but it mostly stayed at 64 so as my OCD rules my life I’m trying to figure out what has changed, I’m not sure I will chase 1-2 mph especially since I’m going slower than I’m actually going but just looking for an answer.

I have not changes my cold tire pressure and it’s usually 29-30 but I’m sure I have some wear on the tires but my thinking is the speedometer would be going the other direction with tire wear but maybe I’m confused .

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[oldcjguy]

As your tires wear or pressure gets low your speedo will read faster than your actual speed. As your tires wear they are getting shorter. Also low pressure makes them shorter. The jeeps computers are programmed with a tire height. From there it can calculate revolutions of the tire per mile. The computers know home many revs your wheels are making based on gearing, engine rpm and transmission gearing. So, n revs of the tire = x miles. Picture rolling a bicycle tire and a wheelbarrow tire. The bike tire covers more distance for each full revolution. If you did 10 revolutions in 10 seconds the bicycle tire gets you further down the road in the same amount of time for those same 10 revolutions.

If it bugs you and you have a jscan you can just reset your tire height. The most accurate way is to measure the distance from the ground to the middle of the axle with warm tires. Always measure from the ground because the weight of the vehicle on the rim will squash the tire some from the center. This measurement gives you your effective radius and takes into account tire pressure, vehicle weight, tread wear, etc... Then double that number for your effective diameter. The effective diameter is what the computer needs to calculate revolutions per mile.

 

[WranglerMan]

@oldcjguy i had it backwards, I was thinking as the tires are wearing my thought was the speedometer would read less than actual speed not more so with that said would I just say change my tire size to a larger tire in JSCAN ?

 

[oldcjguy]

@oldcjguy i had it backwards, I was thinking as the tires are wearing my thought was the speedometer would read less than actual speed not more so with that said would I just say change my tire size to a larger tire in JSCAN ?

Change to a smaller size. The larger the tire diameter, the faster the speedo will read at a given engine RPM. (With a larger size you're covering more distance per rotation) Try measuring the radius from the ground up and doubling like I posted above. That should get you pretty close. I'm not familiar with jscan so I don't know what the sizing options are.

 

[WranglerMan]

Change to a smaller size. The larger the tire diameter, the faster the speedo will read at a given engine RPM. (With a larger size you're covering more distance per rotation) Try measuring the radius from the ground up and doubling like I posted above. That should get you pretty close. I'm not familiar with jscan so I don't know what the sizing options are.

Thank you so much, I’m fairly mechanical but can’t seem to wrap my head around this so thanks for steering me in the right direction, I will set it smaller by say 0.5” and see what happens but being off less than 2 mph is not a bad thing especially since it indicates I’m going faster than I actually am as Constables here in Houston cut you zero slack for speeding.

 

[WranglerMan]

@oldcjguy i went into JSCAN today and I initially dropped the size from 35.70 I think to 35.25” and that got me pretty close but my OCD needed to tweak it a bit more so I went down to 35” and it’s spot on from 75 mph and down

After I was all done I changed a few other things in the IBS section and then reset the TCM since I adjusted the tire size and went for a 30 min highway drive and its spot on and no issues.

Thanks for the help..

 

[SoCal]

measure the actual height of your tire by putting a level on highest point of the tire and then measure that to the ground....My brand new 35" RG's with 36 psi were actually only about 34.2" high so I set it at actual height and it is pretty accurate

 

[WranglerMan]

measure the actual height of your tire by putting a level on highest point of the tire and then measure that to the ground....My brand new 35" RG's with 36 psi were actually only about 34.2" high so I set it at actual height and it is pretty accurate

This is what I initially did but then I never revisited this later after some tire wear plus deciding to run a lower pressure after doing another chalk test.

In the end I ended up being 3 adjustments down from where I had it originally set and it’s spot on.

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