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Old March 27th, 2008, 06:21 PM   #1
Rocky
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Default What gears you need and How much will my speedo be off.

Gear Ratio
(RPM x Tire Diameter) / (Diff Ratio x Trans Ratio x Transf Case Ratio x 336) = MPH

Crawl Ratio
Trans Ratio x Transf Case Ratio x Axle ratio = Crawl Ratio

P-Metric to Inch Diameter
[(Section Width x Aspect Ratio x 2) / 25.4)] + Rim Diameter = Tire Diameter
Ex 235/75R15

[(235 x 0.75 x 2) / 25.4)] + 15 = 28.87"

Tire Change (Find a new Gear ratio)
(New Tire Dia / Old Tire Dia) x Old Axle Ratio = New Gear Ratio

Speedometer Adjuster (With Oversize Tires)
(New Tire Diameter / Old Tire Diameter) x Speedometer MPH = Actual MPH

Another Great Gear Ratio Calculator

The below table can be used to get a rough idea on gear ratios. The colors represent ideal RPM's at highway speeds (65). For highway cruising and best fuel economy stay towards the yellow (2600 rpm), around town daily driving is color coded green (2800 rpm), and for better towing power or just more 4-low power use the ratios near the red (3100 rpm). These calculations are assuming a manual transmission with a 1:1 ratio. If you drive an automatic your RPMs will be slightly higher, and the opposite is true if you have overdrive (your RPMs will be slightly lower).


Chart and info from Mark I. Medina www.4Lo.com
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Old December 28th, 2010, 10:35 PM   #2
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my question is if you re gear the rear axles do you also need to do the front as well im have 4.10s in the rear of my 4 cylinder yj and im going to a 4.88 because of the 33 inch tires so should i do the front also
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Old January 21st, 2011, 12:11 PM   #3
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i hope you are joking.
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Old January 21st, 2011, 12:34 PM   #4
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Quote:
Originally Posted by gotmudd1992 View Post
my question is if you re gear the rear axles do you also need to do the front as well im have 4.10s in the rear of my 4 cylinder yj and im going to a 4.88 because of the 33 inch tires so should i do the front also
you should beable to get away with just doing the left side (driver) and it will work great
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Old December 11th, 2011, 02:00 AM   #5
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Well your right about the numbers changing with an automatic and overdrive , I have an 09 JK with 513's with 37" tires and at 70 mph Im cruising at 2400 2500 rpm with plenty of power , but when I turn off the overdrive it sky rockets to 3200 3300 !! Thank god for that over drive !! And in low gear (4-1 Rubicon transfer) I have more power than I need !!
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Old December 11th, 2011, 09:27 AM   #6
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In my 06 LJ with an auto, I went 4.56's with 33's for a couple years. It seemed about right for the road but I really needed 4.88's for the trail. Now that I've moved to 35's, the 4.56's are not cutting it. Really wished I went to 4.88's (5.13 would have been better but that's just way too small of a pinion in a D30).

But my other problem on the trail is that I'm running a 4:1 low range. It's just tooooooo low for Michigan, so I'm trying to do everything in high range. And every once in a while I go to low range.
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Old January 2nd, 2012, 08:56 AM   #7
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i wonder if a 4 cyl would make a difference on this chart.
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Old January 10th, 2012, 04:12 PM   #8
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Your rpms are the same with any motor what affects rpms is transmission axle ratio and tire size
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Old February 28th, 2012, 02:31 AM   #9
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Also keep in mind it is dependent on what motor you have, I diesel will love lower rpm's and still make power. however a 4cyl will be useless in that rpm range.
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Old March 5th, 2012, 09:11 PM   #10
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Quote:
Originally Posted by shawn View Post
In my 06 LJ with an auto, I went 4.56's with 33's for a couple years. It seemed about right for the road but I really needed 4.88's for the trail. Now that I've moved to 35's, the 4.56's are not cutting it. Really wished I went to 4.88's (5.13 would have been better but that's just way too small of a pinion in a D30).

But my other problem on the trail is that I'm running a 4:1 low range. It's just tooooooo low for Michigan, so I'm trying to do everything in high range. And every once in a while I go to low range.

Wow Shawn I always asumed you had a d44 in that
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Old May 30th, 2012, 09:39 AM   #11
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This might help for the jeep guys who want to calibrate their speedo
http://www.quadratec.com/jeep_knowle...article-46.htm
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Old October 28th, 2013, 09:22 PM   #12
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i just threw 33s on my xj its pretty sluggish so ill be going to 4.10s this weekend.. i drive almost 2000 miles a week other wise id go with a higher gear
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