Yet another speedo gear question - JeepForum.com

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post #1 of 26 Old 06-15-2017, 12:03 PM Thread Starter
secretnarwhal
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Yet another speedo gear question

Hopefully just a simple question for you guys:

I'm still on 30 inch tires and 3.73 gears, as was the factory setup for my Jeep. My speedo is consistently fast by around 8% (reads 25 when I'm going 22-23, reads 80 when I'm going about 74) Checked all the speedo gear charts I could find and they all say I need a 34-tooth gear. Pulled it out yesterday and the gear that's in there has 34 teeth and is in great shape. There isn't any sort of speedo healer installed. Anyone have any idea why my speedo would read fast if everything I've checked is as it should be?

I might just go ahead and buy a 35 tooth gear to try to bring it back down but I was curious if anyone had any new thoughts/cheap solutions.


'00 TJ, 4.0, 5-speed, 114k
'91 YJ, 2.5L 5-speed, 116k *sold*
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post #2 of 26 Old 06-15-2017, 12:44 PM
JEK3
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I'm assuming you compared your speed to GPS. I think I remember mine being a little high also, before I screwed it up by putting bigger tires on. I've heard that a lot of vehicles do read a little high for liability purposes (so the factory isn't to blame for your speeding tickets or crashes due to excessive speed), but the odometer is generally spot-on (legal concerns with warranties, etc.). Does your odometer read accurately, or is it also about 8% high?
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post #3 of 26 Old 06-15-2017, 02:02 PM Thread Starter
secretnarwhal
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I'm just trying to find something to fix/correct because most cars only read 1-2 mph over at highway speeds, so it seems strange that I'm off by so much on a stock system.

I have not checked the odometer yet though - I had assumed it would be the same as the speedo. I'll run it against the GPS on the ride home today

'00 TJ, 4.0, 5-speed, 114k
'91 YJ, 2.5L 5-speed, 116k *sold*
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post #4 of 26 Old 06-15-2017, 02:29 PM
2jhanna
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If I remember correctly, my 30"/3.73 had a 34 tooth.

I find the best way is a GPS/smart phone app, using odometer for at least 20 miles on the open interstate.

jhanna
denton county, tx
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post #5 of 26 Old 06-15-2017, 03:13 PM
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Quote:
Originally Posted by secretnarwhal View Post
I'm just trying to find something to fix/correct because most cars only read 1-2 mph over at highway speeds, so it seems strange that I'm off by so much on a stock system.

I have not checked the odometer yet though - I had assumed it would be the same as the speedo. I'll run it against the GPS on the ride home today
Your question can only be answered without "guessing" on everyone's part if you know the Rolling Radius of your tire.

There's a potentially big difference between what's written on your tire's sidewall and the actual number of revolutions the tire makes in a mile. That's what is referred to as Rolling Radius, Rolling Diameter and Rolling Circumference.

Measure from the center of the wheel hub straight down to the ground. That's your Rolling Radius. Multiply by 2, that's your Rolling Diameter. Multiple by 3.14, that's your Rolling Circumference. Divide by 12, and then divide that into 5280, that's your revolutions per mile.

Revolutions per mile is how the speedometer industry calibrates a speedometer.

Tell us your Rolling Radius and the tooth count will be known for sure.
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post #6 of 26 Old 06-15-2017, 04:07 PM
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Here's a spreadsheet I created years ago. It calculates the driven gear with more precision (less rounding) so that you can see what you will end up with, (or have) based on a "real" gear count and Rolling Diameter of your tire.

Also notice the tire Revs/Mile. This number is what tire manufacturers publish for the tire's spec sheet and it is used to program OEM vehicle ECMs.

Look up your tire on the mfg's web site, you'll see the tire Revs/Mile. That can also be used with this chart instead of measuring your tire.

One will also notice when you study any tire's spec sheet that the tire Revs/Mile do not accurately calculate based on the published tire diameter. And that's because there's a difference between Diameter and Rolling Diameter and Rolling Diameter is used to calculate the tire's Rev/Mile.

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post #7 of 26 Old 06-15-2017, 06:27 PM
01yz426
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Quote:
Originally Posted by secretnarwhal View Post
Hopefully just a simple question for you guys:

I'm still on 30 inch tires and 3.73 gears, as was the factory setup for my Jeep. My speedo is consistently fast by around 8% (reads 25 when I'm going 22-23, reads 80 when I'm going about 74) Checked all the speedo gear charts I could find and they all say I need a 34-tooth gear. Pulled it out yesterday and the gear that's in there has 34 teeth and is in great shape. There isn't any sort of speedo healer installed. Anyone have any idea why my speedo would read fast if everything I've checked is as it should be?

I might just go ahead and buy a 35 tooth gear to try to bring it back down but I was curious if anyone had any new thoughts/cheap solutions.
Can't you just have your speedometer recalibrated for your specific tire/gear size??? Had this done in my SuperDuty...
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post #8 of 26 Old 06-15-2017, 07:13 PM
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Originally Posted by 01yz426 View Post
Can't you just have your speedometer recalibrated for your specific tire/gear size??? Had this done in my SuperDuty...
Not in a TJ.

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post #9 of 26 Old 06-16-2017, 09:21 AM Thread Starter
secretnarwhal
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Thanks for the help guys. After checking the odometer last night it was spot-on (.3 miles more than the GPS after 21 miles). I haven't measured the actual diameter of my tires yet, but I bet that's the reason. I didn't think about brand new tires varying significantly from their nominal size. I'll measure them later, but I'll probably be going to 32's sometime in the future anyway, so might as well just stick with the speedo being fast until then

'00 TJ, 4.0, 5-speed, 114k
'91 YJ, 2.5L 5-speed, 116k *sold*
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post #10 of 26 Old 06-16-2017, 09:34 AM
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Originally Posted by secretnarwhal View Post
Thanks for the help guys. After checking the odometer last night it was spot-on (.3 miles more than the GPS after 21 miles). I haven't measured the actual diameter of my tires yet, but I bet that's the reason. I didn't think about brand new tires varying significantly from their nominal size. I'll measure them later, but I'll probably be going to 32's sometime in the future anyway, so might as well just stick with the speedo being fast until then
.3 off for 20 miles equates to 1.5 miles for 100 miles, or 1.5%. That's as close as you can get.

If you think about the speedo gear with 30-35 teeth, one tooth is approx 3%. So if you changed speedo gear, you would still be off 1.5% in the other direction.

That's why I said use the odometer and not the speedometer, it's hard to watch a needle. Next time you're on a road trip like 100 miles of interstate, check it against a GPS and odometer.

jhanna
denton county, tx
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post #11 of 26 Old 06-19-2017, 11:48 AM Thread Starter
secretnarwhal
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Checked the tires, and the manufacturer spec has them listed as 29.8 inches in diameter. A little surprising because that couldn't throw the speedo off much at all. More importantly, the revs/mile they have listed as 699, which, according to ftgiles's chart (super handy btw - thanks ftgiles) still calls for a 34 tooth gear.

So I suppose I still don't know exactly what could be causing it other than a somewhat large factory offset. Either way, I've made up my mind that I'll live with it until I get me some 32's .

Thanks guys

'00 TJ, 4.0, 5-speed, 114k
'91 YJ, 2.5L 5-speed, 116k *sold*
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post #12 of 26 Old 06-19-2017, 12:20 PM
2jhanna
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Originally Posted by secretnarwhal View Post
Checked the tires, and the manufacturer spec has them listed as 29.8 inches in diameter. A little surprising because that couldn't throw the speedo off much at all. More importantly, the revs/mile they have listed as 699, which, according to ftgiles's chart (super handy btw - thanks ftgiles) still calls for a 34 tooth gear.

So I suppose I still don't know exactly what could be causing it other than a somewhat large factory offset. Either way, I've made up my mind that I'll live with it until I get me some 32's .

Thanks guys

Not sure what youre getting at here, are you unhappy that you are within 1.5%?

Did you even read my post #10?

jhanna
denton county, tx
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post #13 of 26 Old 06-19-2017, 12:29 PM
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Not sure what youre getting at here, are you unhappy that you are within 1.5%?

Did you even read my post #10?
I think he's accepting it's as good as it can be by leaving it alone.
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post #14 of 26 Old 06-19-2017, 03:23 PM Thread Starter
secretnarwhal
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Yes I saw your post 2jhanna. I'm happy with where the odometer is, because that's as good as it can reasonably be. I just find it strange that the factory setup for the speedo would display 5-6mph fast at highway speeds. Most vehicles read 1-2mph over and 5-6 seems excessive to me. I was considering throwing in another gear to correct the speedo at the cost of an odometer that would be off in the opposite direction (1.5% off to -1.5%).

But I've grown accustom to judging speed by looking at the tach anyway. So I'm leaving it alone and dealing with it until I make further modifications down the road.

'00 TJ, 4.0, 5-speed, 114k
'91 YJ, 2.5L 5-speed, 116k *sold*
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post #15 of 26 Old 06-19-2017, 04:06 PM
Jerry Bransford
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Quote:
Originally Posted by 01yz426 View Post
Can't you just have your speedometer recalibrated for your specific tire/gear size??? Had this done in my SuperDuty...
Quote:
Originally Posted by biffgnar View Post
Not in a TJ.
I'm not sure what you were thinking but the TJ's speedometer can of course be recalibrated to different tire size and axle ratio combinations by replacing its snap-in speedometer gear. Only the Rubicon doesn't have that option.

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

It of course isn't always going to give a precision mph indication due to minor tire diameter differences but the right speedometer gear gives a mph indication that is good enough to work with and to remove any possible excuse for a speeding citation.

When you have a choice, buy American made.

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