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Old 06-08-2010, 11:02 AM   #1
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Speedometer calabration

I have a 2003 jeep wrangler rubicon. I think the speedo is off about 3-4 mph. I can live with this but was wanting to know if anyone has any experience correcting this type of issue.

Thanks

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Old 06-08-2010, 11:04 AM   #2
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Depends...

1. 3-4mph over your actual speed, get bigger tires.

2. 3-4 mph lower than your actual speed, reduce tire size.

but who would want to do option 2?

I would guess a mechanic can do it, but as long as you remember that your speedometer is off, why waste the money?

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Old 06-08-2010, 11:11 AM   #3
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You can buy replacement speedometer gears from various parts houses. The down side is it costs about $70 for the gear + installation. My speedo is about 5 over @60, but I'm going to wait to correct it until after I regear.

Edit: Make that 5 under.. I'm doing 65 by my GPS when my speedo reads 60.
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Old 06-08-2010, 11:21 AM   #4
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Is it constantly off 3-4 mph? Or is that at 60? So at half the speed, you're only off 1-2 mph. If that's the case, you will need either a programmer or possibly a dealership to correct it, but that won't be cheap.

If it's always 3-4mph off, then you can pull the needle off and reset it at 0.
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Old 06-08-2010, 11:24 AM   #5
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sub-question...what dictates the tackometer? (RPM reading) does the speedometer control it or are they seperate reading mechinism?
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Old 06-08-2010, 11:40 AM   #6
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Keep in mind your odo is off too... Did you upgrade to bigger tires? Swapping speedo gears is the fix as stated above. If you're ok with it, then don't bother... Also, as mentioned above if it is 3-4 mph faster than you are really going then it was already adjusted for larger size tires, then taken back to closer to stock size....
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Old 06-08-2010, 11:41 AM   #7
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Changing a speedometer gear is a simple 5 minute job. Check with a local 4x4 shop they might have a gear laying around, i got one for about 40 bucks. Search "speedometer gear" for instructions.
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Old 06-08-2010, 11:47 AM   #8
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well I didnt think about this but my tires are worn down about 1/2 inch and that could be throwing my speed off by about 3 or 4 mph. i physcally measured my tires and they are 30inches tall. at factory LT245-75R16 are around 30.5 so that could be my issue
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Old 06-08-2010, 12:02 PM   #9
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The RPMs is a separate mechanism

Speedometer Calibration Program - Method 1

Here is a calculator for the speedometer gear
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Old 06-08-2010, 12:03 PM   #10
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im going about 3-4 slower than what my speedo is saying
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Old 06-08-2010, 12:15 PM   #11
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the reason i asked if the tach is seperate thing was my RPM calulator for the size of gears could be off. I was just making sure I still have 4.10 gears in it and not 3.73s

This jeep was owned by someone else and i have no idea what they did to it...I'm pretty use its been lifted but it now has stock size tires on it. (LT245 75R16) I found a calculator online that would give me the proper RPM reading at a certain MPH with my gear ratio, final tranny gear and tire height input. All the numbers look right to me. at 60 MPH im cruising around 2100-2200 RPMs.
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Old 06-10-2010, 10:59 AM   #12
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Here's the deal, first of all Rubicons don't use a speedometer gear, they have a sensor that counts impulses from a ring on the output shaft of the transfer case and the speedometer is run by the computer. To correct inacurate reading you will need to buy a electronic speedometer correction device. I have one on my 03 Rubicon from "Dakota Digital". There are a number of manufacturers that make these devices and if you do a search on the internet you can find several brands. Good luck and PM me if you have questions.
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Old 06-10-2010, 11:00 AM   #13
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Originally Posted by InfernoGirl View Post
Keep in mind your odo is off too... Did you upgrade to bigger tires? Swapping speedo gears is the fix as stated above. If you're ok with it, then don't bother... Also, as mentioned above if it is 3-4 mph faster than you are really going then it was already adjusted for larger size tires, then taken back to closer to stock size....
Is this actually true? I was under the impression it was not.
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Old 06-10-2010, 11:16 AM   #14
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Originally Posted by 03rubi View Post
Here's the deal, first of all Rubicons don't use a speedometer gear, they have a sensor that counts impulses from a ring on the output shaft of the transfer case and the speedometer is run by the computer. To correct inacurate reading you will need to buy a electronic speedometer correction device. I have one on my 03 Rubicon from "Dakota Digital". There are a number of manufacturers that make these devices and if you do a search on the internet you can find several brands. Good luck and PM me if you have questions.
so this is a device that stays on your rubi? I'm just wondering if tire wear has anything to do with mine being off about 3-4 MPH. the speedo will say im doing 60MPH but two of my GPSs tell me im doing about 56-57 MPH. The tires look to be warn down about 1/2 inch
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Old 06-10-2010, 11:37 AM   #15
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Originally Posted by Ray - San Jose View Post
You can buy replacement speedometer gears from various parts houses. The down side is it costs about $70 for the gear + installation. My speedo is about 5 over @60, but I'm going to wait to correct it until after I regear.

Edit: Make that 5 under.. I'm doing 65 by my GPS when my speedo reads 60.
Mine it the exact same 60 is really 65
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Old 06-10-2010, 12:05 PM   #16
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Is this actually true? I was under the impression it was not.
No, I just make stuff up........
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Old 06-11-2010, 11:14 AM   #17
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Here's the deal, first of all Rubicons don't use a speedometer gear, they have a sensor that counts impulses from a ring on the output shaft of the transfer case and the speedometer is run by the computer. To correct inacurate reading you will need to buy a electronic speedometer correction device. I have one on my 03 Rubicon from "Dakota Digital". There are a number of manufacturers that make these devices and if you do a search on the internet you can find several brands. Good luck and PM me if you have questions.

Truth, and usually you can have a dealership or tuner reprogram the VCM to have the correct ratio as well.

I believe the jeep uses a 40 tooth reluctor wheel located on the tail housing of the transmission for 2wd and on the end of the transfercase for 4wd.
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Old 06-11-2010, 11:43 AM   #18
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Hey wench, where'd you find that calculator? Mine is off too by about 7-9 I have the little dodad thingy majingy that once I hook it up it'll correct it, but I figure as long as I have a GPS why bother. but who know's I might get a hair up my who ha this weekend and fix it.
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Old 06-11-2010, 11:44 AM   #19
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Oh and by the way, YES those are the tech terms used on the dodads box!!

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