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Old 06-30-2011, 09:56 PM   #1
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I got a question....

with the new gears (4.56's) and axles my speedo is exactly 20 MPH off.........will this effect my odometer? i filled the jeep up and before the swap i would only get around 160 miles per tank (3.07 gears) now ive fot 170 on the trip meter and still over 1/4 tank? is this too good to be true?

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Old 06-30-2011, 09:59 PM   #2
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Yes, it will run up your milage also. Swap out the speedo gear to (I think it a 36 tooth or 39 tooth), I can't remember which. It's a 5-10 minute job.

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Old 06-30-2011, 10:01 PM   #3
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crap!! i think ill keep it alone...makes me feel better lol
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Old 07-01-2011, 12:47 AM   #4
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Nothing quite like feeling better about one's self through subterfuge and deceit...... :P
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Old 07-01-2011, 01:32 AM   #5
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A couple of things. One, you can't really get an accurate calculation of your mpg until you get it fixed. Two, your odometer is going to be off so you won't know exactly how many miles are really on your jeep for doing oil changes or if you need to sell the Jeep. Third, you cannot be off exactly 20 mph all the time since we are talking about a difference in ratios.

Here is an example.

Lets take a completely stock jeep with 3.07 gears and change them to 4.56 gears. When the speedometer says 30mph it is actually doing 20mph(a difference of 10mph). At 40mph the actual speed is 27mph(13mph off). At 50mph it's off by 17mph, at 60mph it is off by 20mph and at 70mph it is off by 23mph and so on.
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Old 07-01-2011, 02:11 AM   #6
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Quote:
Originally Posted by Coyote_97TJ View Post
A couple of things. One, you can't really get an accurate calculation of your mpg until you get it fixed. Two, your odometer is going to be off so you won't know exactly how many miles are really on your jeep for doing oil changes or if you need to sell the Jeep. Third, you cannot be off exactly 20 mph all the time since we are talking about a difference in ratios.

Here is an example.

Lets take a completely stock jeep with 3.07 gears and change them to 4.56 gears. When the speedometer says 30mph it is actually doing 20mph(a difference of 10mph). At 40mph the actual speed is 27mph(13mph off). At 50mph it's off by 17mph, at 60mph it is off by 20mph and at 70mph it is off by 23mph and so on.
You should be able to get an accurate calculation of gas mileage if you know the percentage your tire size/gearing is off... take your odometer reading, multiple by 1.xx (xx being your percentage represented as a decimal). Divide by number of gallons used. Am I right...?
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Old 07-01-2011, 02:26 AM   #7
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Is there a thread about aftermarket tire size vs ratio to get the correct speedo gear?
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Old 07-01-2011, 02:57 AM   #8
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Quote:
Originally Posted by HULIO View Post
Is there a thread about aftermarket tire size vs ratio to get the correct speedo gear?


I just bought a new gear tonight to compensate for my 33's so hope that helps...lol
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Old 07-01-2011, 04:13 AM   #9
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THE CORRECT SPEEDO GEAR IS 39 TOOTH WITH 33in. TIRES..
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Old 07-01-2011, 04:18 AM   #10
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lol you should see how far off my camaro is, 393 gearing with stock sized tires.. if i take it to work (13 miles each way so 26 total) it adds about 100 miles to my odometer :P
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Old 07-01-2011, 05:53 AM   #11
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I just replaced mine 28's to 31's (30T-27T)

My mileage is perfect. It was off about 3-5% (hard to tell) I just filled up for the first time, same mpg? I thought I would see a little difference but I did not. Going to run another tank and see what happens.
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Old 07-01-2011, 10:08 AM   #12
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Awesome. Thanks. I saved that chart. So that's the gear I need to buy. I'll have to see about getting one of those
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Old 07-01-2011, 10:41 AM   #13
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Quote:
Originally Posted by s3nt3nc3d View Post
You should be able to get an accurate calculation of gas mileage if you know the percentage your tire size/gearing is off... take your odometer reading, multiple by 1.xx (xx being your percentage represented as a decimal). Divide by number of gallons used. Am I right...?
LOL, I'm not that good at math so for me it is a lot easier to just get the right gear rather than trying to do all that math while at a rest stop or where ever. And I have no idea if you are right.



Here are a couple of links to calculators that I use in case anyone is interested.

4Lo.com :: Final Gear Ratio, Crawl Ratio, Tire Size Calculators

Gear Ratio Calculator

And here is another gear ratio chart just because.

4Lo.com :: Gear Ratio & Tire Size Chart

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