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  #1  
Old November 18th, 2020, 19:57
maxbraketorque maxbraketorque is offline
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Ring & pinion ratio vs tire size

All the ring and pinion ratio vs tire size charts for the XJ simply adjust the R&P ratio to offset the increased tire size on a 1:1 ratio. So if I were running a 31" tire which is about 10% large diameter than stock by the numbers on the tires, then I would change from a 3.55:1 to a 3.90:1 R&P. And for 33" tires, it would be 4.10:1. However, I've now seen plenty of posts where people with 33" tires are running 4.56 and 4.88:1 R&P ratios. Since the TC has a 4LO option, I don't understand the point of substantially undergearing. Does rock crawling or navigating over very difficult terrain benefit from doing this even with an auto trans?
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Old November 18th, 2020, 20:22
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EricsXJ EricsXJ is offline
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Re: Ring & pinion ratio vs tire size

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Originally Posted by maxbraketorque View Post
All the ring and pinion ratio vs tire size charts for the XJ simply adjust the R&P ratio to offset the increased tire size on a 1:1 ratio. ...
I don't know what charts you are looking at but this is wrong and not based on reality. Yes for 33's the ideal gears for most people is 4.56. My ideas as to why a straight 1:1 adjustment does not work is because in real life you are not just increasing tire diameter. There are other factors that come into play such as weight. Bigger tires are heavier... along with other additions commonly made at the same time as the bigger tires such as aftermarket bumpers, sliders, skids, cages, spares, tools, and gear. There is also a vertical change both due to taller tires but also with an accompanied lift. This probably has more effect at highway speeds with wind drag rather than slow crawling, but it is a factor.
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Last edited by EricsXJ; November 18th, 2020 at 20:31.
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Old November 19th, 2020, 08:19
maxbraketorque maxbraketorque is offline
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Re: Ring & pinion ratio vs tire size

ok. I wouldn't have thought people running 33's and higher would care much about driving faster than 60 mph where wind speed matters more, but additional vehicle weight and increased inertial mass of the wheels does make sense for raising up the cruise RPM to higher power portion of the powerband.

I'm on 31's now with stock 3.55 R&P, but I'm considering swapping up to a higher numeric R&P ratio. Trying to figure out what makes sense. At some point in the future I may want to go to 33" tires, and I don't want to do another R&P swap at that time.
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Old November 19th, 2020, 08:35
RCP Phx RCP Phx is online now
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Re: Ring & pinion ratio vs tire size

To answer the other half of your question, the stock 231 and 242 TC's only have a 2.72 reduction in low range and isn't considered much. I have had a 4.1 in my NP231 for twenty years and a NV OR241 and will never go back to stock. Compare your final drive ratio's and you'll see there is a major difference.
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Old November 19th, 2020, 10:05
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Re: Ring & pinion ratio vs tire size

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Originally Posted by RCP Phx View Post
To answer the other half of your question, the stock 231 and 242 TC's only have a 2.72 reduction in low range and isn't considered much. I have had a 4.1 in my NP231 for twenty years and a NV OR241 and will never go back to stock. Compare your final drive ratio's and you'll see there is a major difference.
There's gotta be a thread on xfer case gear reduction? Brand names, etc.

Found this with little effort however.

http://www.4x4wire.com/jeep/tech/xfer/tera4_1/
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Old November 19th, 2020, 10:08
maxbraketorque maxbraketorque is offline
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Re: Ring & pinion ratio vs tire size

Thanks. My guess would be that low of a gearing is mostly needed for very extreme terrain. I'm more in the range of basic double-track. I'll likely go with 4.10 R&P.
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Old November 19th, 2020, 10:18
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Re: Ring & pinion ratio vs tire size

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Originally Posted by maxbraketorque View Post
ok. I wouldn't have thought people running 33's and higher would care much about driving faster than 60 mph where wind speed matters more, but additional vehicle weight and increased inertial mass of the wheels does make sense for raising up the cruise RPM to higher power portion of the powerband.

I'm on 31's now with stock 3.55 R&P, but I'm considering swapping up to a higher numeric R&P ratio. Trying to figure out what makes sense. At some point in the future I may want to go to 33" tires, and I don't want to do another R&P swap at that time.
My XJ is on 33's and 4.56's with a 5 speed. I do care about driving faster than 60mph as I drive it to places to wheel. Averaged around 70mph driving from Houston to Moab a few weeks back.
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Old November 19th, 2020, 12:26
gearwhine gearwhine is offline
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Re: Ring & pinion ratio vs tire size

I'm waiting on some new 4.56 gears right now. 3.55s on 31s with an AW4 currently, and it is pretty horrendous. It bogs big time. I can't wait to get the new gears in.

I have driven 3.07s on 33s with a manual for a while before going to 4.56. That set up more doable than the auto as there is more control, but it still sucked.

4.56 was still extremely solid highway speeds on 33s. I, personally, would never go to a lower numerical gear than 4.56 on 33s even if it's a highway only XJ. Which is why I'm choosing 4.56 for 31s this time around.
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Old November 19th, 2020, 12:55
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Re: Ring & pinion ratio vs tire size

Typical gear calculation: New tire size / stock tire size x current gear ratio = New gear ratio (ex: 33" tire / 28" stock tire x 3.54 stock gear = 4.17 (4.10 ratio).
Just calculating percentage change in tire size diameter is why most gear charts are incorrect though and end up leaving you under geared (under powered).

Tire diameter increase does not account for the tire weight increase which is considerable when it is added as rotational mass. Cruising rpm's will increase due to this added weight as the engine will have to use more power to move them. Then consider most off-road vehicles include added weight in the form of bumpers, rock sliders, roof racks, frame stiffeners, tube flares, recover gear, etc, etc.

A better calculation:
33" tire / 28" tire x 2000 stock cruising rpm = new goal rpm. (33/28*2,000= 2,357). Using that calculation a 4.56 gear ratio will put your rpms at 2,263 at 65mph. 4.88 at 2,422 rpm's at 65mph. You can see why these are the more regularly run and recommended gear ratios.

Last edited by WAZZUXJ; November 19th, 2020 at 12:59.
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Old November 19th, 2020, 13:49
maxbraketorque maxbraketorque is offline
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Re: Ring & pinion ratio vs tire size

Thanks much. Sounds like if I'm going to stick with 31" tires for sure, 4.10 is a good choice, but if I feel pretty certain that 33" tires are in my future, I should go with 4.56.
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Old November 19th, 2020, 14:03
75SV1 75SV1 is offline
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Re: Ring & pinion ratio vs tire size

http://www.grimmjeeper.com/gears.html

That one gets post in most of these types of discussions. I'm on 4.10 GR and an Auto. I did have 31's. It was a good ratio. I'm on 32's not. A bit doggy. It could be other issues. Also, depends on what you want out of your ride. Do have a build going on with 3.73 auto and 30.5's (metric). Seems good, but haven't run it on the highways. I have a 231, but plan a 241TC. I could see say a 3.73 and a 4:1 TC in this setup. Hiway milage, then lower TC for offroad.
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Old November 22nd, 2020, 14:25
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Re: Ring & pinion ratio vs tire size

Quote:
Originally Posted by maxbraketorque View Post
if I feel pretty certain that 33" tires are in my future, I should go with 4.56.
Yes.
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Old November 22nd, 2020, 21:36
maxbraketorque maxbraketorque is offline
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