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Discussion Starter · #1 ·
I've always referred to the gear ratio chart as my Wrangler bible for bigger tires, but I'm wondering now if it needs to be updated? I'm running 285/65R18 (32.8" x 11.9") Nitto Trail Grappler tires on a 2015 JKU Sahara 3.6L Auto w/ 3.73 gears. According to the chart I should be humming at 2,207-2,276 RPMs at 70mph.

However I've noticed that my RPM is consistently higher 2,350-2,400 RPMs running this tire size. I snapped the attached pic this morning cruising along a flat highway with minimal (if any) wind conditions. Since I did not ProCal yet, 70mph is the equivalent to 69mph on the speedometer (verified by an app on my iPhone and the head unit nav).

Originally I thought a 33" tire was my max without needing to re-gear but now I'm wondering if I could actually go bigger (34-35") and still stay in the optimal power band.

Curious what everyone else is seeing in real world driving...
 

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Discussion Starter · #4 ·
I've been telling people that for a while I have 315 70 17 just under 35s and have no problems with 373 gears and that's at 6500 feet altitude.
Good to know! Thanks!
 

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What is the actual measured diameter of your tire when mounted on your wheel?

Since there's quite a bit of variation between marketed tire size and actual measured tire size when mounted, the gearing chart can't possibly be accurately done in terms of marketed tire size. I assume the chart is based on actual measured tire size. If this is correct, then what is commonly referred to as a 33" tire would more likely be considered a 32" tire when using this chart.
 

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Discussion Starter · #7 ·
What is the actual measured diameter of your tire when mounted on your wheel? Since there's quite a bit of variation between marketed tire size and actual measured tire size when mounted, the gearing chart can't possibly be accurately done in terms of marketed tire size. I assume the chart is based on actual measured tire size. If this is correct, then what is commonly referred to as a 33" tire would more likely be considered a 32" tire when using this chart.
Just took a tape measure to it and I measure 32 3/4" (32.75") left to right and 32 3/8" (32.375") from pavement to top of the tire. So the Nitto marketed size is pretty close in my case.
 

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I've been telling people that for a while I have 315 70 17 just under 35s and have no problems with 373 gears and that's at 6500 feet altitude.
Amen and agreed. It's a great combo and comes it just above 34"s! have the same 3.73 and 315/70/17.
 

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I have always assumed the math that generates those charts don't take into account the actual running diameter of those tires sizes. The input is what the charts states, 32, 33, 34, 35, etc. So, yes. The chart is "wrong". My 285/70/17 Duratracs are 33 inches tall, give or take a few tenths. But on the Jeep, they roll like 31.5's or 32's.

Of course, that's an assumption. I've never verified that myself but since each tire/wheel/air pressure set up is different and power is a subjective thing, I always used the chart as a guide, not a rule. If you drive 37's on 3.21's and like it, who am I to argue?
 

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Whaaat, the gear chart lies???? :jawdrop:

No, not really. The printing on tire sidewall, speedo and tach are more likely the sources of lies and deception.
 

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Discussion Starter · #12 ·
Even using the mounted diameter, the chart is ~100rpm off which gives me more confidence for 34" tires :)
 

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I know I've been saying it (my face is a shade of Blue) and I doubt I'm the only one.
Gear/Tire/Trans - Nirvana is a matter of opinion and as much Art as it is a science.
It's great to have a reference/starting point but bottom line is : One Man's heaven is another Man's Hell (insert "Wo" ahead of Man where applicable)
It just is a matter of opinion in the end.
 

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I think the chart is pretty accurate. With 35" MTRs, my 4.10s are not adequate in these hilly and mountainous areas and at altitude. My 35" MTRs now measure 34.25" after half life and at 26-28 PSI. If this 3.6 had decent torque, the 4.10 would be perfect. So if they managed to put 30-40 more ft/lbs of torque and similar HP, the gearing chart wold be a bit outdated. I am holding off on rehearing because I only have 11,000-12,000 miles on it but I also keep thinking about a LS conversion so no point dropping 2K+ on are gear till and if I am sure I will not do the conversion. People at sea level and flat land are happier with 4.10s since they don't have to deal with altitude and steep terrain. Here, most do 4.56 and some do 4.88 with the 3.6 with 35s. 4.10s just make the 3.6 scream and flail if you want to maintain 65-75 up the mountain highways and passes.
 

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I've always referred to the gear ratio chart as my Wrangler bible for bigger tires, but I'm wondering now if it needs to be updated? I'm running 285/65R18 (32.8" x 11.9") Nitto Trail Grappler tires on a 2015 JKU Sahara 3.6L Auto w/ 3.73 gears. According to the chart I should be humming at 2,207-2,276 RPMs at 70mph. However I've noticed that my RPM is consistently higher 2,350-2,400 RPMs running this tire size. I snapped the attached pic this morning cruising along a flat highway with minimal (if any) wind conditions. Since I did not ProCal yet, 70mph is the equivalent to 69mph on the speedometer (verified by an app on my iPhone and the head unit nav). Originally I thought a 33" tire was my max without needing to re-gear but now I'm wondering if I could actually go bigger (34-35") and still stay in the optimal power band. Curious what everyone else is seeing in real world driving...

The chart close but not perfect as each tire can be different size, some jeeps heavier.
I have 33" Mickey Thompson MTZ which are about 32.5. I had 3.21 gears which I ran 1 year and while tolerable on the road they were brutal off road for crawling up any steep rock.
I have regeared to 4.56 as I plan to do 35 when tires are gone, I was worried about to much gear but I love the way she crawls with 33" tires. Gas mileage on the other hand sucks but only summer vehicle now so not worried. I will eventually get bigger tires.
My RPM with 3.21 was around 1900 with 4.56 it's 2800. I have 2012 with 3.6 and this motor has power especially when geared correctly.
I just went on. Trip about 500km and got 16.6 MPG normally around 15 MPG

Before 3.21 gears

View attachment 2352897


After 4.56

View attachment 2352937
 

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I know I've been saying it (my face is a shade of Blue) and I doubt I'm the only one. Gear/Tire/Trans - Nirvana is a matter of opinion and as much Art as it is a science. It's great to have a reference/starting point but bottom line is : One Man's heaven is another Man's Hell (insert "Wo" ahead of Man where applicable) It just is a matter of opinion in the end.
Ding ding! This right here..... Use the chart as a guide and not a rule that must be followed. Everyone has way too many variations from build to build for the chart to be definitive for everyone building a JK.
 

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should you use the aired-down diameter of your tires when referencing this chart?
 
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