How to recalibrate your speedometer after changing tire sizes or gears

Well, I hate ask a dumb question: Just re-geared to 5.13, every chart I read says 43 tooth. Novak chart says those charts are inaccurate, yet they do not show any data for 5.13 gears.
42RLE:
5.13gears:
33/17" Tires that measure ~31.5" w/~27psi

Do we agree with 43 tooth?
 
S.McArthur said:
Well, I hate ask a dumb question: Just re-geared to 5.13, every chart I read says 43 tooth. Novak chart says those charts are inaccurate, yet they do not show any data for 5.13 gears.
42RLE:
5.13gears:
33/17" Tires that measure ~31.5" w/~27psi

Do we agree with 43 tooth?
Click to expand...
Best way to find out is to take a measurement with GPS and compare to what your speedo reads. You also need to know what speedo gear you have now. You should have a 34 tooth but I wouldn't bet the price of a speedometer gear on it. I would inspect the gear you have now and take at least one measurement that way there is no guesswork involved. If you just follow the chart you'll probably be fine. From what you are suggesting your speedometer should now read 76 when you are going 60 on GPS, does that sound right?
 
pagrey said:
Best way to find out is to take a measurement with GPS and compare to what your speedo reads. You also need to know what speedo gear you have now. You should have a 34 tooth but I wouldn't bet the price of a speedometer gear on it. I would inspect the gear you have now and take at least one measurement that way there is no guesswork involved. If you just follow the chart you'll probably be fine. From what you are suggesting your speedometer should now read 76 when you are going 60 on GPS, does that sound right?
Click to expand...
Yes, from 45MPH+ is a ~10MPH difference (used for on the fly), gradually increasing. Took it on the highway this morning @70MPH the gauge reads 85.

I need to get my RPM at 60MPH and I can use the formula. Estimating an RPM, I'm between 41-44.7.
 
If you have a 34 tooth gear and your speedometer reads 85 at 70 then

43 tooth gear will make your speedometer read 67 at 70
42 tooth gear will make your speedometer read 69 at 70
41 tooth gear will make your speedometer read 70.5 at 70

That's if you have a 34 tooth gear
 
pagrey said:
If you have a 34 tooth gear and your speedometer reads 85 at 70 then

43 tooth gear will make your speedometer read 67 at 70
42 tooth gear will make your speedometer read 69 at 70
41 tooth gear will make your speedometer read 70.5 at 70

That's if you have a 34 tooth gear
Click to expand...
I gotcha, I’ll pull it and give it a count.
 
I installed the 34 tooth, now my speedo reads 69 at 70 on the gps...I just dont know if it is worth changing for 1-2 miles per hour. But I am also not of the "it must be perfect" generation...
 
astjp2 said:
I installed the 34 tooth, now my speedo reads 69 at 70 on the gps...I just dont know if it is worth changing for 1-2 miles per hour. But I am also not of the "it must be perfect" generation...
Click to expand...

But you did check your speedometer with GPS

Might have to revoke your generation X card for that.
 
Wish I thought to check where the numbers where lined up prior to removal, but here is a good example of why you need to line the numbers up on your speedo gears. The red and blue are from the speedo gear and the gear it meshes with on the output shaft. The red speedo gear "clearanced" itself which is why you see the red flakes.
I'm assuming this happened when the PO had the SYE installed, all I wanted to do was fix the leaking case, now I'm damn near rebuilding the TC.
The FSM says the numbers on the adapter should be at the 6 O'Clock position, @Chris , you mention 5 O'clock, typo or did you find that the 5 position was better?
IMG_1137.JPG
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S.McArthur said:
Wish I thought to check where the numbers where lined up prior to removal, but here is a good example of why you need to line the numbers up on your speedo gears. The red and blue are from the speedo gear and the gear it meshes with on the output shaft. The red speedo gear "clearanced" itself which is why you see the red flakes.
I'm assuming this happened when the PO had the SYE installed, all I wanted to do was fix the leaking case, now I'm damn near rebuilding the TC.
The FSM says the numbers on the adapter should be at the 6 O'Clock position, @Chris , you mention 5 O'clock, typo or did you find that the 5 position was better?
View attachment 102272View attachment 102273View attachment 102274
Click to expand...

I never actually had to do this on mine as I simply used the SpeedoHealer electronic module.
 
[QUOTE="S.McArthur, post: 387617, member: 7678"
The FSM says the numbers on the adapter should be at the 6 O'Clock position, @Chris , you mention 5 O'clock, typo or did you find that the 5 position was better?

[/QUOTE]
I did mine about a month ago, and put it at the 5 o'clock position, and it worked fine.
 
374gatorgirl said:
I unfortunately have a 3.07 gear ratio (2002 TJ automatic, 4.0) and 33" tires (yes I know, not ideal- bought it this way without proper knowledge). Since the chart is blank for this combination, does that mean I cannot change the speedometer gear? I can't afford to regear at this time but the speedometer being 5 MPH off drives me crazy. Am I just stuck with it until I regear?
Click to expand...

I unfortunately have the same thing. 3.07 with 33" tires. 4.0 manual, i purchased a 26 tooth and it matched up perfectly with GPS and phone. no off to do my new member post.
 
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Thanks for all the information. I pulled mine out and counted teeth to be sure of what I had in there. With 32 inch tires and 34 teeth I'm exactly 5 mph under at 70 mph. I tossed my little commute data set into the formula and get

60/70*34 = 31.57
50/54*34 = 31.48
40/43*34 = 31.62

So which way should I round?

Doing the math the other direction I'll be more accurate with a 32 tooth gear and it would keep me 1 mph under GPS speed when reading 70 mph. If I did the math right. If I go with a 31 tooth gear, I would be going 1.2 mph over when reading 70 mph, but it would line up if I ever changed tires and went a little bigger (because who would go smaller?).

Maybe I'll look harder at the other thread on rewiring the Healthtech speedometer calibrator and just do that once. I'm an engineer so accuracy is incredibly important before I glance down at analog gauges while driving in traffic. :ROFLMAO:

Here are the spreadsheet results.

31.57​
32​
31​
SpeedoActual, MPH
Actual, MPH​
20​
19.73​
-0.27​
20.37​
0.37​
30​
29.60​
-0.40​
30.55​
0.55​
40​
39.46​
-0.54​
40.74​
0.74​
50​
49.33​
-0.67​
50.92​
0.92​
60​
59.19​
-0.81​
61.10​
1.10​
70​
69.06​
-0.94​
71.29​
1.29​
80​
78.93​
-1.08​
81.47​
1.47​
​
​
​
​
​
 
InOmaha said:
So which way should I round?
Click to expand...
Both are pretty close. Keep in mind that a 32" tire could change about 2% over its life and those gears are only 3% different. When you get to this point things like where your tires are in their life become important. The factory probably picks a number part way between new and totally worn out for their default so the speedometer drifts through the correct spot as the tires wear. I'd might consider the same thing if I was in your situation but to tell you the truth I'd probably get the 32 just because it's cheaper. Even with an electronic corrector the drift is going to happen as the tire wears although it sounds like you'd have fun with one. If you buy two gears you could have bought the corrector.
 
Makes sense. These tires are brand spanking new with the nubs just wearing off. So as they age, they'll slowly get smaller and accuracy of the 32 tooth speedometer gear should improve. :)

I also aired down the tires after reading threads on tire pressure. The previous owner had them at 38 psi, so I dropped them down to the 29 psi on the door for now. That will affect the actual rolling radius too.
 
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