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post #1 of 21 Old 11-27-2006, 04:52 PM Thread Starter
Gav'spurplez
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Speedo Calibration

bought my wrangler a few weeks ago and the tires are larger than from the factory.

30x9.5x15 i think are the stock tires and the ones on the jeep are 31x10.5x15

so does the speedo need to be calibrated ??

just wondering, they look fairly new so they have not been on the jeep for too long,


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post #2 of 21 Old 11-27-2006, 04:54 PM
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I would not install a new speedometer gear for just 31" tires. If you go bigger, you definitely want a new gear, and here is how:

http://www.4x4xplor.com/speedo.html


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post #3 of 21 Old 11-27-2006, 04:54 PM
Jerry Bransford
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The speedo is probably still close enough so you wouldn't need to calibrate it. Got a GPS you can use or borrow to test against? Otherwise, it's only $15 or so for a new speedo gear and 5-10 minutes to install.

When you have a choice, buy American made.
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post #4 of 21 Old 11-27-2006, 06:34 PM Thread Starter
Gav'spurplez
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thanks for your information.

would like to go with bigger tires,
but will need a lift, and i do not think that is in the budget right now

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1 of 95 in Bright Purple Met.
Basic bolt-ons and 17in DRs.
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post #5 of 21 Old 11-27-2006, 06:45 PM
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Quote:
Originally Posted by Gav'spurplez
thanks for your information.

would like to go with bigger tires,
but will need a lift, and i do not think that is in the budget right now
31x10.5" tires fit fine on a TJ with stock suspension. Down the road you can add a Rubicon Express Budget Boost for $200 and it will really set those 31s of nice.


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post #6 of 21 Old 11-27-2006, 07:15 PM
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I have 32" tires and 3.73 gears and got pull over last week for speeding. I played dumb and the nice officer gave me a warning. At 50 he said I was going 57 (speed limit was 45 where he clocked me = 17 over). Hard for me to believe mine is 7mph off at 50.

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post #7 of 21 Old 11-27-2006, 07:28 PM
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Quote:
Originally Posted by CJ-TJ
I have 32" tires and 3.73 gears and got pull over last week for speeding. I played dumb and the nice officer gave me a warning. At 50 he said I was going 57 (speed limit was 45 where he clocked me = 17 over). Hard for me to believe mine is 7mph off at 50.

well, if i'm correct the stock tire size on my 97 SE is 225/75/r15, which works out to be about a 28.

now that i'm running 32's, there should be quite a difference. I am not sure if it is correct, but in diameter that works out to be about a 14% difference. Does this mean my speedometer is off by 14% as well? I dont think the speedometer is directly related to the diameter of the tire, isnt it based on circumference, and doesnt it have to do with the gearing of the jeep also?

if i use a calculator online such as this one
http://www.4lo.com/calc/gearspeedo.htm

you only enter your original size, and your new size. this tells me that when going 40 mph = 45.7mph
50 mph = 57 mph
65 mph = 74.3

this works out to be the same difference as the tires, 14%.

is this correct? seems like faulty logic to me, theres way more involved than just tire diameter, isnt there? or am i just completly wrong?

-Andrew

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post #8 of 21 Old 11-28-2006, 06:24 AM
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Quote:
Originally Posted by Dr. Marneaus
theres way more involved than just tire diameter, isnt there? or am i just completly wrong?
what are you asking? It is directly related to rie diameter, as you need the diameter to measure circumfrence. Circumfrence of the tire increases exponentially as the diameter increases.

Consider a 28 inch tire has roughly a circumfrence of 87.92" (28x3.14)
a 31 inch tire has roughly a circumfrence of 97.34"
a 32 inch tire has roughly a circumfrence of 100.48

you can see that a 32 inch tire travels an extra 13 or 14 inches per rotation than a 28 inch tire.

A mile has 63360inches. So, a 28 inch tire will spin 720.6 times over the course of a mile, where a 32 inch tire will spin 630.4 times. With a rough difference of 14 percent. (630.4 / 720.6 = 13.6%.. since all numbers were rounded we can assume its closer to 14%)

Ofcourse gears and all that also affect it, but were only talking about changing tires here.

I think my math is pretty close, but it is early..

Anyhow, Dr. Marneaus how do you find the SE with 32inch tires? You didnt regear? Hows your power on and off the trail... notice increased slipping on the clutch when you start?

Im considering getting larger tires for next season.

Last edited by Combatsylvania; 11-28-2006 at 06:34 AM.
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post #9 of 21 Old 11-28-2006, 06:47 AM
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My 2000 TJ came with 30x9.5x15 tires and 3.73 gears, and there was a couple of MPH difference between the speedo and my Garmin GPS, now with 31x10.5x15 tires it is right on (less than 0.5 MPH difference at 60).
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post #10 of 21 Old 11-28-2006, 06:52 AM
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a few more questions

I recently regeared my Jeep to 4.88. It is a 4cyl and I am running 35s. I have looked at several charts about what speedo gear to get, but do they apply to both 4cyl and 6cyl? If so, I think I need the 39 tooth gear...is that correct? I upgraded to the terflex ess sye, will any speedo from say q-tec fit? thanks.
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post #11 of 21 Old 11-28-2006, 07:48 AM
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the speedo gear plugs in to the TCase so your SYE shouldnt effect it. Id assume you need one to match your TCase not the engine.
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post #12 of 21 Old 11-28-2006, 07:56 AM
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a guess i was wondering because the rear tailshaft housing (which holds the speedo gear) was replaced when I did the SYE kit. I just wanted to make sure it fit and I can't get in touch with teraflex.
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post #13 of 21 Old 11-28-2006, 10:24 AM
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Quote:
Originally Posted by kercher
a guess i was wondering because the rear tailshaft housing (which holds the speedo gear) was replaced when I did the SYE kit. I just wanted to make sure it fit and I can't get in touch with teraflex.
Yes, a standard speedometer gear will work fine. And yes, a 39 tooth is correct and will get your speedo dead-nuts on with that tire and gear ratio combination.

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post #14 of 21 Old 11-28-2006, 10:45 AM
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Quote:
Originally Posted by Combatsylvania
... Circumfrence of the tire increases exponentially as the diameter increases. ....
Actually, it increases linearly with the diameter, not exponentially. For every inch of diameter that you add, you add one inch times PI in circumference.

Other than that statement, the math looks correct.

Marc L - '05 WHITE Rubicon Unlimited
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post #15 of 21 Old 11-28-2006, 10:55 AM
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Quote:
Originally Posted by ziaholic
Actually, it increases linearly with the diameter, not exponentially. For every inch of diameter that you add, you add one inch times PI in circumference.

Other than that statement, the math looks correct.
you are correct sir... my bad
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