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Old 12-25-2012, 05:16 PM   #1
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Odometer readings, how to fix?

With 30-9.50 15 tires, my speedometer reads two miles per hour too fast. I checked it with my GPS.

So how am I to read my odometer, or interpret it, to get a right read? For example, I have 115,000 miles on it supposedly. I must not have that many, but how do I calculate to figure it out? Also, in a trip of 100 miles, is there some way to calculate it?

It doesn't seem to matter if I am going 35 or 70, my speedometer always reads 2 miles an hour too fast.

Thanks.
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Old 12-25-2012, 05:41 PM   #2
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Not sure how yours is showing the same mph offset regardless of speed. Usually it is a percentage difference.


I just went from 31's to 33's and purchased a kit from blue monkey motorsports. It is a kit that includes the device from healtech along with a plug n play harness. It is the best, least intrusive solution I could find.

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Old 12-26-2012, 08:15 AM   #3
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2 mph, I would not worry about it.

(Note, only a Rubicon need an electronic speedometer correction. All other TJ's it is a mechanical gear but it will be a percentage as indicated with different size tires)
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Old 12-26-2012, 08:55 AM   #4
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Just to note- it's only 2mph off at one speed. A speedometer is not a linear instrument, therefore the faster you go, the more it reads incorrectly. It's inaccurate by a percentage, not by an actual unit of measure (mph).
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Old 12-26-2012, 09:35 AM   #5
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The odometer will also be off by the same percentage as the speedo, at least for the amount of time you have been running bigger tires. Find out how many miles you have been running that size tire, figure the percentage your speedo is off and then multiply according. This will tell you how many miles to add/subtract from your odo reading.

Example: Mine is 10% off running 33's... speedo reads 50, Im actually going 55. Spedo reads 70, I'm actually going 77. Assuming the PO ran the 33's from day one, I would have 66K miles on my jeep. Odo reads 60K X 10% = add 6K miles to the current reading.
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Old 12-27-2012, 07:23 AM   #6
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Well, I will put my GPS back in the Jeep and look at it again. Maybe I was wrong.
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Old 12-27-2012, 08:31 AM   #7
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The easy way is to measure your tire diameter and buy the appropriate speedo gear....about a 5 minute install and about $40 here: Search Results - Quadratec
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Old 12-28-2012, 01:36 AM   #8
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Quote:
Originally Posted by Water Dog View Post
The easy way is to measure your tire diameter and buy the appropriate speedo gear....about a 5 minute install and about $40 here: Search Results - Quadratec
This was my thought. Jeep speedometer gears
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Old 12-28-2012, 01:58 AM   #9
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How can I tell what gears I'm running? (Please don't kill me...I looked...I PROMISE!)
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Old 12-28-2012, 05:05 AM   #10
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There should be a tag on your front and rear differential bolts with the ratio. (It may need to be cleaned to read it)
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Old 12-28-2012, 11:14 PM   #11
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Quote:
Originally Posted by Neil F. View Post
There should be a tag on your front and rear differential bolts with the ratio. (It may need to be cleaned to read it)
THANKS!
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Old 12-29-2012, 01:58 PM   #12
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How can I tell what gears I'm running? (Please don't kill me...I looked...I PROMISE!)
If you can't find a tag, jack up the rear end and have someone turn a tire one full revolution while you count the number of revolutions your drive shaft makes. If the drive shaft makes a little over 3 revolutions, you have 3:07 gears, if it makes about 3 3/4 revolutions, you have 3:73, a little over 4 revs is 4:10 and so on.

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