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Discussion Starter #1 (Edited)
The title says it: Does airing down affect final drive ratios? I would think not since, even though the 'rolling diameter' changes, the circumference of the tire doesn't change. Am I thinking correctly on that?

Also, and I realize it would be under-geared, has anyone run something like Pritchett Canyon on 37s in a 2012 with 4.10s, the 4:1, and an auto? If so, tolerable?
T
 

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I would think the 2012 with 4.10's and an auto would be fine with 37's. No way I'd try anything older than that with 37's and 4.10, even the 6spd, but I think you'll be fine. That being said, I'm talking about numbers on paper because I've never wheeled there or driven a 2012 auto. :D

As for your first question, I think the effective diameter of the tire does change since it's aired down. Just how much that affects the drive ratio, I'm not sure. Maybe I'll do a test run next time I go out to wheel.
 

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I'd say it changes it. However it will take a pretty drastic pressure change to make a negligible difference in the final drive ratio.

I would definitely not recommend this to try to "keep a lower final drive ratio" due to the very high rolling resistance problem you will make. Any "lower height gain" you will absolutely eat up in rolling resistance.


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Discussion Starter #4 (Edited)
I'd say it changes it. However it will take a pretty drastic pressure change to make a negligible difference in the final drive ratio.

I would definitely not recommend this to try to "keep a lower final drive ratio" due to the very high rolling resistance problem you will make. Any "lower height gain" you will absolutely eat up in rolling resistance.
No - not my intent at all. I'm thinking about it in terms of running a larger set of tires off-road only.

I've built a number of rigs in my 18 yrs of wheeling; been through the tow rig days with a trailer queen rockcrawler, etc ... all that is gone now and the JK is a much milder build that is seeing dual duty as some DD, family camper, and trail truck. I'm perfectly content with 35s/4.10s and the auto. It works very well on and offroad.

However, I'd like to pick up a set of 37s on beadlocks for off-road only and was thinking about the effect on gearing. That got me thinking - 'would I need to regear since the they'd be run at 10psi (ie - "decreased" running diameter) virtually all the time?' I've also figured gearing starting with tire diameter ... but that is to determine circumference. That led to me to realize that, in the end, it comes down to tire circumference, not 'diameter'.

But, I need a logic check and thus my quesion ... Thinking about it today, even at lower psi, the circumference of the tire "shouldn't" change ... but, I feel like I'm missing something because even 8 psi change on my 35s at road pressure changes the dynamic diameter and can throw the speedo off unless I 'Procal' it.

Ultimately, I'm just trying to figure out if I could 'get by' on a trail like Prichett, for example, on 37s, 4.10s, and auto on a 2012 (knowing that 4.10s and 35s work fine ... final drive ratios come out to nearly exactly the same as stock 32s and the standard 3.73s).

T
 

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Final drive ratio is a mere claculation and equals top gear ratio times axle ratio.

To determine other ratios in whatever gear simply multiply the transmission gear of choice by the axle ratio.

Crawl ratio is similar in so far as being a mere calculation. In this case it's 1st gear ratio times transfer case ratio times axle ratio.

So tire diameter doesn't affect either - but that doesn't mean real world performance is unaffected.

An auto gives a huge benefit to real world results, particularly with crawl ratio since you can modulate throttle prior to lock up, getting the torque you need but not the speed you don't.

With 4.10's I'm assuming you have a Rubicon. That means 4:1 transfer case. The WA580 has a pretty low 1st gear, a huge advantage for the 2012's. 3.59 1st gear ratio X 4 (transfer case ratio) = 14.36, that times 4.10 (axle ratio) = 58.876. That is a fine crawl ratio with 37's.

My own (5.7VVT Hemi, 545rfe transmission, 4.88 axles, 37's) is 1st gear ratio 3.00 x 4 = 12; 12 x 4.88 = 58.56

I have a ton more torque with the Hemi, but the torque converter in your Jeep will allow your 3.6 to rev enough to produce the torque you need for great control.

FWIW, I run at about 15psi off road, on BFG, know for being a shorter tire for nominal diameter.

Also fwiw, final drive ratio for your Jeep: .83 x 4.10 = 3.403; and for mine: .67 x 4.88 = 3.2696, so your is geared lower on the highway...

JPK
 

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Discussion Starter #10 (Edited)
Final drive ratio is a mere claculation and equals top gear ratio times axle ratio.

To determine other ratios in whatever gear simply multiply the transmission gear of choice by the axle ratio.

Crawl ratio is similar in so far as being a mere calculation. In this case it's 1st gear ratio times transfer case ratio times axle ratio.

So tire diameter doesn't affect either - but that doesn't mean real world performance is unaffected.

An auto gives a huge benefit to real world results, particularly with crawl ratio since you can modulate throttle prior to lock up, getting the torque you need but not the speed you don't.

With 4.10's I'm assuming you have a Rubicon. That means 4:1 transfer case. The WA580 has a pretty low 1st gear, a huge advantage for the 2012's. 3.59 1st gear ratio X 4 (transfer case ratio) = 14.36, that times 4.10 (axle ratio) = 58.876. That is a fine crawl ratio with 37's.


FWIW, I run at about 15psi off road, on BFG, know for being a shorter tire for nominal diameter.

Also fwiw, final drive ratio for your Jeep: .83 x 4.10 = 3.403; and for mine: .67 x 4.88 = 3.2696, so your is geared lower on the highway...

JPK
JPK-

Thanks - you didn't really directly answer what I was attempting to ask, but kinda' re-made my point in a round about way.

Let me clarify a bit -

First, I probably should have said "final/'realized' overall effective gear ratio" instead of "final drive ratio". "Final drive ratio" can be ambiguous depending on what site or forum one is on and because, as you stated, simply put FDR=Tranny ratio X axle ratio, "final drive ratio" wasn't the best choice of words on my part.

Second, my use of that term, however technically incorrect, stems from knowing that, all things being equal (ie- the calculated final drive ratio), shorter tires will give a lower 'realized/final' gear ratio than taller tires on a given vehicle.

So, with my current set-up on 35s (which I'm perfectly content with on and offroad), I am naturally concerned about having appropriate gearing when throwing on a set of 37s for dedicated off-road only use.

You touch on my point with crawl ratio. While you say ~59 is a good crawl ratio for 37s, my prior rigs have been higher (closer to having a 2012 auto with 4.88s, for example - CR of ~70).

So, given that, it got me thinking about what effect air pressure has ... lowering air pressure 'lowers' the weighted radius of the tire and, thus, the overall 'weighted' tire diameter. But, the circumference doesn't change ...

So, the point of my "logic check" was to simply verify what I was thinking; mainly, that despite running larger tires aired down (in the past I've run 37s at 8-12psi on a similarly weighted rig), ideally I'd still need to regear to hit optimal gearing off-road for that size tire because the circumference of an aired-down 37" tire is larger than a 35" tire at road pressure, despite 'weighted' diameters being the same/similar.

T
 
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