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Old 03-16-2014, 12:57 PM   #1
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Gear help, sorry.

Second owner of 2012 JKU. Judging by the photo and tire/gear chart, I have 4.56-4.88 gears but build sheet has 3.73. Diablo trinity programmer is also set a 3.73 gears. 35" tires. Thoughts?
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Old 03-16-2014, 02:49 PM   #2
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What gear are you in?

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Old 03-16-2014, 03:11 PM   #3
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Besides the gear as NWBroco asked, what size tires do you have?

Easiest thing is to deprogram your JK and run the test again.
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Old 03-16-2014, 04:20 PM   #4
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Or get a rear wheel off the ground and spin it around twice. That would give you your ratio.

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Old 03-16-2014, 07:07 PM   #5
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Originally Posted by nwbronco View Post
Or get a rear wheel off the ground and spin it around twice. That would give you your ratio.

Bob K.
That is actually the correct way to do it. Mine was the lazy method.
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Old 03-17-2014, 12:50 AM   #6
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Assuming sixth gear, the chart says more like 5.38 to 5.86, doesn't it? Or am I reading it wrong? Also appears to be a manual trans.

Another thought: did you check the speedometer with a GPS?
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Old 03-17-2014, 07:39 PM   #7
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Manual, 6th gear, 35", 3.6. I will give GPS speedo a shot, but if its accurate, this goes against the chart, right?
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Old 03-17-2014, 10:03 PM   #8
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Originally Posted by GafferGail View Post
Second owner of 2012 JKU. Judging by the photo and tire/gear chart, I have 4.56-4.88 gears but build sheet has 3.73. Diablo trinity programmer is also set a 3.73 gears. 35" tires. Thoughts?
2012+ JK's will go in to Limp mode if the proper gears are not programmed in to the tranny controller. So somoen has programmed the correct ratio in.

I have 5.13 gears and when I am at 70mph I am at 3000 rpm in Over Drive. I would say you have 5.13 gears.

A friend of mine has 4.88's and he runs about 2500 RPM @ 70mph.

If you reprogram for 3.73 gears then take a test drive you wont get out of 3rd gear. You can do this until you get to the right ratio and it doesn't go in to limp mode I guess.
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Old 03-18-2014, 12:47 AM   #9
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Originally Posted by gorillahydrographics View Post

2012+ JK's will go in to Limp mode if the proper gears are not programmed in to the tranny controller. So somoen has programmed the correct ratio in.

I have 5.13 gears and when I am at 70mph I am at 3000 rpm in Over Drive. I would say you have 5.13 gears.

A friend of mine has 4.88's and he runs about 2500 RPM @ 70mph.

If you reprogram for 3.73 gears then take a test drive you wont get out of 3rd gear. You can do this until you get to the right ratio and it doesn't go in to limp mode I guess.
That only applies to automatics. His is a manual.
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Old 03-18-2014, 09:45 AM   #10
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Ahhh..ok...i missed that in the later post
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Old 03-18-2014, 11:03 AM   #11
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Manual, 6th gear, 35", 3.6. I will give GPS speedo a shot, but if its accurate, this goes against the chart, right?
My suggestion now that I'm not on my phone and I can type a little better is to do an experiment.

Get your GPS and get on the highway and into 6th gear. Pick a speed and get your GPS to register that speed consistently. Read the tach.

Go find out the exact advertised size of the tire you're running. Not just the calculated size of the tire. Go to the manufacturer's website and find the OD (Outside Diameter) measurement they list for that tire.

Go to my gear ratio calculator: Gear Ratio Calculator

Pick the following:

Transmission: NSG370 Close Ratio (2012+ Jeep Wrangler)
Transfer Case: NP241 (NVG241OR if you have a Rubicon)
Underdrive: None
Axle Gear Ratio: 3.73 to start
Tire Size: Select inch and type in the exact measurement for the tire

Go down to the last set of numbers, the "Road RPMs at given speed in miles per hour" chart. Set the speed in one of the columns to match the speed you were going on your GPS.

Check the RPM in 6th gear. Change the gear ratios between 3.73, 4.10, 4.56, and 4.88 until you find the closest RPM without going over what your tach says. That will be the actual gear ratio you're running.

Keep in mind that the actual tire diameter depends on a lot of little things including air pressure, tread wear, etc. The tire is usually just a little bit smaller than the advertised OD. My 315/75R17 Goodyear Duratracs are advertised at 34.4" OD but I had to enter 33.48" via my programmer to get the speedometer to be accurate.
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Old 03-19-2014, 11:25 AM   #12
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This is my first computer controlled Jeep, so I am not to big on technology. Somehow the tuner lost my tire size, but that wasn't the major issue. I GPS'd the speedo to find out I was about 5-6 miles off. Put 4.56 gears in the tuner and we are within .5 mph. Thanks for the chart. I need to crack the cover on the front and rear to see what else the owner put in. Used to have an 84 CJ7, but my 79 CJ7 is the toy. 350, th400, d300, CUCV 60/14, 36 irok's SOA with XJ springs in rear, RE 1.5 in in the front. About 103 wheelbase... it goes fast and it goes slow.
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Old 03-19-2014, 12:31 PM   #13
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best way to measure true tire diameter is with a small level and tape measure. level out from center of the hub, measure to the ground and double. THAT'S your actual tire diameter.

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