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Old 02-14-2008, 12:11 PM   #1
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recalibrate '06 LJ speedo?

Okay, do I need a new gear behind the dash to go from 30" to 32" tires - or is it a computer flash done from the port under the dash. I've gotten both answers from different sources.

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Old 02-14-2008, 01:31 PM   #2
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If your LJ is not a Rubicon, then you recalibrate your speedometer by simply swapping the speedometer gear which is located in a housing bolted onto the transfer case. Here's how... http://www.4x4xplor.com/speedo.html

You only need to know your axle ratio (probably 3.73) and your tire size to select the correct speedometer gear tooth count. For 32" tires and a 3.73 axle ratio, you'd want a 32 tooth count speedometer gear. It's a 5 minute procedure to swap the gear, the gear snaps in and out of the housing.

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Old 02-15-2008, 11:39 AM   #3
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Did that very thing when I regeared differentials and went to 33" tires. Used the chart Jerry mentions above and it too just minutes to replace. Most of them (speedo gears) come with a rubber o-ring, make sure and replace that too. I checked my mileage against my GPS unit and it was never more that 1/2 mph over the entire speed range.
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Old 02-16-2008, 01:55 PM   #4
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Thanks. That's what I thought, but the stealership confused me by telling me it was done with a computer flash and then I noticed I have a port under the dash.
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Old 02-16-2008, 01:57 PM   #5
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With my 32's Im about 2 mph off from 30's.
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Old 02-16-2008, 02:03 PM   #6
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With my 32's Im about 2 mph off from 30's.
Huh. Not hardly worth worrying about then. 2mph at what speed? 60?
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Old 02-16-2008, 02:13 PM   #7
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Huh. Not hardly worth worrying about then. 2mph at what speed? 60?
At 30,
at 60, 4 mph off
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Old 02-16-2008, 03:18 PM   #8
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with 30s vs 32s your speedo will be reading 6% slower. so if your speedo is reading 50, u'll be going ~53mph
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Old 02-16-2008, 04:07 PM   #9
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I was at 62/63 and my speedo said 60. That was with the cruise set at 60. I wouldnt care if it was 20 mph off. Anything over 80 in a Jeep is just plain scary. My Mustang has a new 140 speedometer and the needle is still set on the old 85. At 55, its says Im doing around between 90-95. I watch the rpms instead.
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Old 02-16-2008, 04:17 PM   #10
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There is a speedometer calculator located here, among others.
http://www.rocky-road.com/calculator.html that lets you put in Old tire size, new tire size and gear ratio and will automatically figure the correct reading of your speedometer.
If you have 3.73 gears and you go from a 30" tire to a 32" tire your speed will be off by 4 mph at 60.
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Old 02-16-2008, 05:18 PM   #11
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with 30s vs 32s your speedo will be reading 6% slower. so if your speedo is reading 50, u'll be going ~53mph
Okay - you're obviously WAY better at math than me. (When I took math in school, we didn't even have calculators!) So..going from 30 to 32 how much would my odo be off after say..3k miles? Reading more miles than actual - or less? (My guess is..less.) Sorry - call me stupid.
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Old 02-16-2008, 06:18 PM   #12
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Okay - you're obviously WAY better at math than me. (When I took math in school, we didn't even have calculators!) So..going from 30 to 32 how much would my odo be off after say..3k miles? Reading more miles than actual - or less? (My guess is..less.) Sorry - call me stupid.
Your tires would be 6.25% larger.
So if you went 3000 miles (according to the odometer) with your 32 " tires. You actually traveled 3187.5 miles.

1.0635 X 3000 = 3187.5
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Old 02-16-2008, 06:25 PM   #13
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Quote:
Originally Posted by Scoob View Post
1.0635 X 3000 = 3187.5
same way to figure out speed.

1.0635 x (what ur speedo says) = real speed

ex. ur speedo SAYS 30 mph

1.0635 x 30 = ~31.9 mph

so when ur speedo reads 30 mph, u're actually going almost 32 mph.

I'd recommend u just do the basics 30, 40, 50, 60 and just to have an idea how fast you really are really going.

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