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hey guys i changed my tires from the stock 225 and 205/70R15 to some 31x10.5's and just out of convienience I'm wondering how to adjust the speedometer so it reads correctly again? ive looked through other posts but found nothing. its mostly for the odometer so i know when to change the oil and stuff but its nice to have a properly functioning set up... anyways hopefully someone knows and its an easy fix

 

thanks in advance

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  • 3 years later...

i was wonderin the same thing, all the calibration shops around my area want too much and i wanna find a way to do it myself other than buyin a new drive gear, is there a way i could set it myself since i'm runnin 225/75/R15 and that's the highest "stock" tires on my MJ?

 

Redwolf

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There is no way to do it yourself other than swapping speedo gears. That usually gets you to within about a +/- 5% accuracy, which translates to being off by about 2-1/2 MPH at 50 MPH. To get it closer than that, speedometer calibration shops play with the return spring on the needle. But they would insist on having the correct speedo drive gear for the tire size before they even start tinkering with the speedo head.

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There is no way to do it yourself other than swapping speedo gears. That usually gets you to within about a +/- 5% accuracy, which translates to being off by about 2-1/2 MPH at 50 MPH. To get it closer than that, speedometer calibration shops play with the return spring on the needle. But they would insist on having the correct speedo drive gear for the tire size before they even start tinkering with the speedo head.

ok, what if ya don't know if ya have the right drive gear?

 

Redwolf

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There is no way to do it yourself other than swapping speedo gears. That usually gets you to within about a +/- 5% accuracy, which translates to being off by about 2-1/2 MPH at 50 MPH. To get it closer than that, speedometer calibration shops play with the return spring on the needle. But they would insist on having the correct speedo drive gear for the tire size before they even start tinkering with the speedo head.

ok, what if ya don't know if ya have the right drive gear?

 

Redwolf

 

Then you take out your drive gear and count the teeth, then look it up in a FSM or an on-line chart. However, to look it up you need to know both the tire size and the gear ratio, and IIRC you aren't certain about your gear ratio.

 

So basically the question would be: was your speedometer reasonably accurate before you changed to the 31" tires? If so, you can back into the speedo gear charts that way. The charts tell you which gear to use for an axle and tire combination. If you count the teeth on your speedo drive gear, then you know what and you know the tire size, so you go into the chart and find the block with that number of teeth with your stock tire size, and that should tell you what axle ratio you have. Then you follow down (or across, depending on how the chart is laid out) to the new tire size, and that tells you how many teeth you need on the new drive gear.

 

As a rule of thumb, going from stock 225/75-15s up to 31x10.50s is a jump of 7 percent. If the original tires on your truck from the factory (which is what the speedo gear will be calibrated for) were 215/75s, the jump will be about 9 percent. All speedo gears cover a range of two or three tire sizes, which is why the speedometer typically isn't exactly accurate even from the factory. So you can get close just by counting the number of teeth on your drive gear and choosing a new gear that has +/- 8 percent fewer teeth.

 

Here's a link to a chart. 225/75 tires are 28.3" in diameter, 215/75s are 27.7" in diameter, so call it 28" for your stock tires. 31x10.50s typically run a bit less than a true 31" but call it 31".

 

http://www.quadratec.com/jeep_knowledgebase/article-46.htm

 

So, for argument, lets say you have 3.55 gears. On the chart, 3.55 gears with 28" tires should have a 35-tooth drive gear. 35 less 8 percent is 32.2, so my backwards  math method would say you need a 32-tooth gear for 31" tires. Looking at the chart, if we find 31" tires and 3.55 gears, it tells us you need ... 32 teeth.

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Actually, with the speedometer gears having around 30 teeth, they will step up or down 3%-3.5% from one to the next. Cut that in half to get to the nearest size you need and you will be within less than 2% off just by changing the gear.

 

That is about the same accuracy as the difference between a new tire and a worn down one.

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i was wonderin the same thing, all the calibration shops around my area want too much and i wanna find a way to do it myself other than buyin a new drive gear, is there a way i could set it myself since i'm runnin 225/75/R15 and that's the highest "stock" tires on my MJ?

 

Redwolf

 

Won't do a thing for the odometer, but that's no biggie on most MJ/XLs - - - - - 

 

If you're happy/satisfied/done messin' with the drive line/tires & want an accurate & inexpensive fix/Speedo display - - - Note where the speedometer needle is pointing when doing a verified 50, 60, 70 or whatever mph - - note rpm - then photo shop or buy an overlay corrected to actual  - - - - Doin' 75 & needle points at 70 for inst? - - Get/fab overlay matching your other gauges with numbers altered accordingly so needle now points to a 75 at same rpm - - Google & many custom jobbers - 1 r 2 here & on NAXJA as well IIRC - - - just sayin'

 

 

:thumbsup:

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According to my research, a 205/75-15 turns approximately 760 revolutions per mile. A 225/75-15 turns approximately 730 revolutions per mile. That's a difference of 4 percent. That means your speedometer would be off by 4 MPH at 100 MPH, or 2 MPH at 50 and 1 MPH at 25.

 

Changing tire size NEVER makes a speedometer read 'X' number of MPH fast or slow at all speeds. It's a proportional difference.

 

Have you checked your actual speed against the indicated speed using a GPS? Do it at different speeds, like 25, 50 and 75 MPH and see what the error is for each speed.

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According to my research, a 205/75-15 turns approximately 760 revolutions per mile. A 225/75-15 turns approximately 730 revolutions per mile. That's a difference of 4 percent. That means your speedometer would be off by 4 MPH at 100 MPH, or 2 MPH at 50 and 1 MPH at 25.

 

Changing tire size NEVER makes a speedometer read 'X' number of MPH fast or slow at all speeds. It's a proportional difference.

 

Have you checked your actual speed against the indicated speed using a GPS? Do it at different speeds, like 25, 50 and 75 MPH and see what the error is for each speed.

none of the roads around here are long enough to test it with the GPS sadly, the only real test i could do it find a cop,

 

Redwolf

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none of the roads around here are long enough to test it with the GPS sadly, the only real test i could do it find a cop,

 

Redwolf

 

???

 

What are you talking about? I dunno what you have, but my wife's Tom Tom GPS gives a constant true speed read-out. My friend down the road didn't have a working speedo in his MJ for a couple of years, and he used his GPS as a speedometer. It gives a constant read-out -- all you have to do is drive. Look at the speedo, look at the GPS, and see how far apart they read.

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none of the roads around here are long enough to test it with the GPS sadly, the only real test i could do it find a cop,

 

Redwolf

 

???

 

What are you talking about? I dunno what you have, but my wife's Tom Tom GPS gives a constant true speed read-out. My friend down the road didn't have a working speedo in his MJ for a couple of years, and he used his GPS as a speedometer. It gives a constant read-out -- all you have to do is drive. Look at the speedo, look at the GPS, and see how far apart they read.

 

oh that kinda GPS, i don't have a tom tom or anything, i have a smart phone and it takes it a while to keep up with someone,

 

none of the roads around here are long enough to test it with the GPS sadly, the only real test i could do it find a cop

 

You live in VB? Did they close the expressway to Norfolk?  C'mon man............ 

i264 is still open but i won't go up there specially in the MJ, that's askin to get bad mileage  :fs1: i'll figure somethin out, i just graduated school 2 years ago and schools bouta start up and i'm still friends with a cop that works at my high school so i could always have him gun me  :thumbsup:

 

Redwolf

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If your police officer friend will agree to check you with radar, be sure to get at least two readings, like at 25 and 50 or 30 and 60. You need to find out if the difference is a constant (like 3 MPH at all speeds) or proportional (like 3 MPH at 30 but 6 MPH at 60).

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In my experience, police radar is worthless for a test. The one time I tried, the police who agreed to help me could not generate a result within 5mph consistently (after 5 attempts), and tried to blame it on my driving (even though I was rock solid at 50mph indicated and with rpm steady on the tach, with a quarter mile lead in to their position). Then when I was out later  that night doing a distance-over-time  test at a steady 35mph indicated in a 45mph zone  - the only car on the road at 2am  - another cop pulled me over on this deserted stretch of road and detained me on the roadside. After he  "snooped" my car through the windows with his flashlight (I told him he did NOT have permission to search it, so he didn't go inside it) and questioned me about my "suspicious behavior" for an hour and a half. Wwearing just shorts and a t-shirt, I was freezing my tail off, standing in a 30mph wind with the temperature at ~50 degrees and falling in the wake of a cold front; I finally got exasperated enough to cuss him and demand that he either take me to jail  (for driving 10mph UNDER the speed limit!!!!!) or let me go, because I was freezing my nuts off standing there in that cold wind!

.

Bah!!!

.

 Sorry,  but in my experience, the police are worse than useless for that sort of help.               

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If your police officer friend will agree to check you with radar, be sure to get at least two readings, like at 25 and 50 or 30 and 60. You need to find out if the difference is a constant (like 3 MPH at all speeds) or proportional (like 3 MPH at 30 but 6 MPH at 60).

ok, i never thought of that, i just thought of one speed but i'll try to get two speed test of 25 and 50 atleast 3 times

 

In my experience, police radar is worthless for a test. The one time I tried, the police who agreed to help me could not generate a result within 5mph consistently (after 5 attempts), and tried to blame it on my driving (even though I was rock solid at 50mph indicated and with rpm steady on the tach, with a quarter mile lead in to their position). Then when I was out later  that night doing a distance-over-time  test at a steady 35mph indicated in a 45mph zone  - the only car on the road at 2am  - another cop pulled me over on this deserted stretch of road and detained me on the roadside. After he  "snooped" my car through the windows with his flashlight (I told him he did NOT have permission to search it, so he didn't go inside it) and questioned me about my "suspicious behavior" for an hour and a half (with me freezing my tail off all the while wearing just shorts and a t-shirt, standing in a 30mph wind with the temperature at ~50 degrees and falling in the wake of a cold front, I finally got exasperated enough to cuss him and demand that he either take me to jail or let me go, because I was freezing my nuts off standing there in that cold wind!

.

Bah!!!

.

 Sorry,  but in my experience, the police are worse than useless for that sort of help.               

well i wanna get tested by a cop since they're the ones that issue the tickets, if it's right on their gun i get no ticket :D

 

Redwolf

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If your police officer friend will agree to check you with radar, be sure to get at least two readings, like at 25 and 50 or 30 and 60. You need to find out if the difference is a constant (like 3 MPH at all speeds) or proportional (like 3 MPH at 30 but 6 MPH at 60).

ok, i never thought of that, i just thought of one speed but i'll try to get two speed test of 25 and 50 atleast 3 times

 

>>>>>In my experience, police radar is worthless for a test. The one time I tried, the police who agreed to help me could not generate a result within 5mph consistently (after 5 attempts), and tried to blame it on my driving (even though I was rock solid at 50mph indicated and with rpm steady on the tach, with a quarter mile lead in to their position). Then when I was out later  that night doing a distance-over-time  test at a steady 35mph indicated in a 45mph zone  - the only car on the road at 2am  - another cop pulled me over on this deserted stretch of road and detained me on the roadside. After he  "snooped" my car through the windows with his flashlight (I told him he did NOT have permission to search it, so he didn't go inside it) and questioned me about my "suspicious behavior" for an hour and a half (with me freezing my tail off all the while wearing just shorts and a t-shirt, standing in a 30mph wind with the temperature at ~50 degrees and falling in the wake of a cold front, I finally got exasperated enough to cuss him and demand that he either take me to jail or let me go, because I was freezing my nuts off standing there in that cold wind!

.

Bah!!!

.

 Sorry,  but in my experience, the police are worse than useless for that sort of help.               

well i wanna get tested by a cop since they're the ones that issue the tickets, if it's right on their gun i get no ticket :D

 

Redwolf

 

.

Sure - they are the ones who issue the tickets - cannot argue that logic, it does have merit.

.

But - my point is that their radar does not yield consistent results. I expect that this is because they do NOT calibrate their equipment every shift, or that their calibration method is flawed, or that the calibration equipment (usually a tuning fork - the ones I have seen are aluminum - bent, dented, and/or nicked from abuse) is flawed - or some combination of all 3 of the above.         And almost never does anyone question the police on their methods... so their word is taken as Gospel, as is the (frequently flawed) reading  given by their radar.

.

That night I recounted above in my previous post I had 2 different police cars check my speed, and not only did their readings not agree with each other, the didn't even agree with their own repeated readings (not even within 5mph). 

.

Furthermore, the police are not there to help you (it is not part of their job), and you could get your friend into trouble  for helping you if he got caught doing so. Bad anyway you slice it.

.

You have gotten tons of good advice so far in  this thread - why not take it, do the  work yourself, and have confidence in the results?                     

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If your police officer friend will agree to check you with radar, be sure to get at least two readings, like at 25 and 50 or 30 and 60. You need to find out if the difference is a constant (like 3 MPH at all speeds) or proportional (like 3 MPH at 30 but 6 MPH at 60).

ok, i never thought of that, i just thought of one speed but i'll try to get two speed test of 25 and 50 atleast 3 times

 

>>>>>In my experience, police radar is worthless for a test. The one time I tried, the police who agreed to help me could not generate a result within 5mph consistently (after 5 attempts), and tried to blame it on my driving (even though I was rock solid at 50mph indicated and with rpm steady on the tach, with a quarter mile lead in to their position). Then when I was out later  that night doing a distance-over-time  test at a steady 35mph indicated in a 45mph zone  - the only car on the road at 2am  - another cop pulled me over on this deserted stretch of road and detained me on the roadside. After he  "snooped" my car through the windows with his flashlight (I told him he did NOT have permission to search it, so he didn't go inside it) and questioned me about my "suspicious behavior" for an hour and a half (with me freezing my tail off all the while wearing just shorts and a t-shirt, standing in a 30mph wind with the temperature at ~50 degrees and falling in the wake of a cold front, I finally got exasperated enough to cuss him and demand that he either take me to jail or let me go, because I was freezing my nuts off standing there in that cold wind!

.

Bah!!!

.

 Sorry,  but in my experience, the police are worse than useless for that sort of help.             

 

well i wanna get tested by a cop since they're the ones that issue the tickets, if it's right on their gun i get no ticket :D

 

Redwolf

.

Sure - they are the ones who issue the tickets - cannot argue that logic, it does have merit.

.

But - my point is that their radar does not yield consistent results. I expect that this is because they do NOT calibrate their equipment every shift, or that their calibration method is flawed, or that the calibration equipment (usually a tuning fork - the ones I have seen are aluminum - bent, dented, and/or nicked from abuse) is flawed - or some combination of all 3 of the above.         And almost never does anyone question the police on their methods... so their word is taken as Gospel, as is the (frequently flawed) reading  given by their radar.

.

That night I recounted above in my previous post I had 2 different police cars check my speed, and not only did their readings not agree with each other, the didn't even agree with their own repeated readings (not even within 5mph). 

.

Furthermore, the police are not there to help you (it is not part of their job), and you could get your friend into trouble  for helping you if he got caught doing so. Bad anyway you slice it.

.

You have gotten tons of good advice so far in  this thread - why not take it, do the  work yourself, and have confidence in the results?                     

 

that is true, i've seen a bunch of different readins on the same rador, and i have takin the thought of doin it myself but part of me is afraid to take that diff cover off for fear that somethin else on my truck will break, that seems to be the case everytime somethin gets messed with,

 

Redwolf

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that is true, i've seen a bunch of different readins on the same rador, and i have takin the thought of doin it myself but part of me is afraid to take that diff cover off for fear that somethin else on my truck will break, that seems to be the case everytime somethin gets messed with,

 

 

Redwolf

 

Since just removing the diff cover doesn't touch any moving parts, it's unlikely you could break anything -- and with an old truck, it's probably long overdue to change out the gear lube anyway. But, if you really don't want to remove the cover, you are left with the turn a wheel and watch the driveshaft method.

 

First off -- do you have an open diff or do you have limited slip? This is important because the test is done differently with a limited slip. With a limited slip (if it's working), you have to jack up both rear wheels. When you turn one wheel, the opposite wheel should turn in the SAME direction. If so, mark the driveshaft with a white or yellow chalk or crayon on the side, where you can see the mark when you're sitting or kneeling at the tire. Get the mark where you can see it, then mark a spot on the tire either at the very top or the very bottom. Now, slowly rotate the tire exactly one revolution, and count how many turns the driveshaft makes. That'll be your ratio.

 

If you have an open differential (no limited slip or locker), you only jack up ONE rear wheel. Keep the other wheel on the ground so it can't turn. Mark the tire and the driveshaft. With an open diff, you rotate the tire through TWO revolutions and count the number of driveshaft revolutions.

 

Either way, be sure to mark the tire so you know you are turning it exactly one or two revolutions. If that's not exact, your count on the driveshaft (which is going to be an approximation at best) will have to be off.

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The Purple People Eater came stock with 205/75R15 tires. I run 235/5R15 on it, and when the speedometer reads 70, I'm going 75 as per my GPS

 

Based on the math of tire specs, a 205/75-15 turns 760 revolutions per mile and a 235/75-15 turns 722 revolutions per mile. 38 divided by 760 gives us a 5% difference. Theoretically, then, at 70 on your speedometer the actual speed should be 73.5. But ... stock speedometers usually read slightly on the fast side, so 70 indicated probably wasn't really quite 70 on the factory tires, and that could easily account for another 1.5 MPH.

 

Anyway, we're in the ballpark, and these examples show how it all works. My '88 XJ came with 205/75-15s, and a long time ago I calculated that the difference from those to 30x9.50s was 6 percent, and the difference from the 205s to 31x10.50s was 11 percent.

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that is true, i've seen a bunch of different readins on the same rador, and i have takin the thought of doin it myself but part of me is afraid to take that diff cover off for fear that somethin else on my truck will break, that seems to be the case everytime somethin gets messed with,

 

 

Redwolf

 

Since just removing the diff cover doesn't touch any moving parts, it's unlikely you could break anything -- and with an old truck, it's probably long overdue to change out the gear lube anyway. But, if you really don't want to remove the cover, you are left with the turn a wheel and watch the driveshaft method.

 

First off -- do you have an open diff or do you have limited slip? This is important because the test is done differently with a limited slip. With a limited slip (if it's working), you have to jack up both rear wheels. When you turn one wheel, the opposite wheel should turn in the SAME direction. If so, mark the driveshaft with a white or yellow chalk or crayon on the side, where you can see the mark when you're sitting or kneeling at the tire. Get the mark where you can see it, then mark a spot on the tire either at the very top or the very bottom. Now, slowly rotate the tire exactly one revolution, and count how many turns the driveshaft makes. That'll be your ratio.

 

If you have an open differential (no limited slip or locker), you only jack up ONE rear wheel. Keep the other wheel on the ground so it can't turn. Mark the tire and the driveshaft. With an open diff, you rotate the tire through TWO revolutions and count the number of driveshaft revolutions.

 

Either way, be sure to mark the tire so you know you are turning it exactly one or two revolutions. If that's not exact, your count on the driveshaft (which is going to be an approximation at best) will have to be off.

gotcha, i'm not sure if i have open or limited split, didn't pay attention to the other wheel when i tried the test the first time, bein that my rear end is a dana 35 could it be only an open or did the dana 35 come in limited slip as well?

 

Redwolf

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