4.56 or 4.88 gears with NSG370 34 inch tires

Huntinangler

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Hi everybody, brand new forum member here, and new to being a forum member in general other than stalking around for good info and insight! Forgive me if this question or info has already been discussed… I had a question about gearing, taking into account transmission and tires, and was hoping for some expert advice. I’ve spent quite a bit of time looking at gear charts and reading information, but still I’m not 100% sure if my calculations are accurate. I recently got amazingly lucky and found a unicorn- bone stock, low mileage, one owner, ‘05 LJ Rubicon with NSG370 6 speed. I sold my other jeep to buy this one, which was a ‘04 TJ Rubicon with AX15 5 speed, 5.5” RE long arm kit, 35” tires, and 4.88 gears. Since I now have a blank slate, I’m planning and starting my new build. Going to go a little different direction with a goal of putting together a “sleeper” of a wheeler that will get me out or through some nasty spots, but be lower to ground and a little more driver friendly for my wife and future 16 year old. Also will be a little more convenient for putting around ranches and hunting the mountains. My plan for this jeep is a 3.5”-4” Long arm kit with true 34” (34.1”) tires. My question for for forum members is with the NSG370 6 speed and 34” tires, should I go with 4.56 or 4.88 gears? I’m having a heckuva time making up my mind. With my 04 Rubicon and higher geared AX15 5-speed / 4.88’s / 35’s, the gearing could’ve been a tad bit lower and I would’ve been happy… especially in first gear around town. I’m just really nervous to spend all the money and get 4.88’s, and be unhappy with it being way too low? I live in the mountains, a lot of 55-65 mph and tiny small towns, but do venture down the mountain and encounter some higher speeds at times. I think if I could get close to stock Rubicon 4.10 31” Tire and 6speed feel and drivability I would be happy. Sorry for the long winded novel, thanks and I really appreciate your insight!!!
 
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The final drive on the 6-speed and 5-speed are virtually the same.

I have 4.88. I have a 5-speed and 35s and if I were doing it again I'd probably go 5.38 since I don't really care about needing to cruise at 75 and would prefer the increased crawl ratio offroad.

But, 4.88 will easily allow you to cruise at 70-75 on the highway and give you a better crawl ratio for the rocks offroad.
 
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@JMT has the 6-speed with 35's and 4.56.

I'm sure he has an opinion.
 
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Between 4.56 and 4.88 ... 4.88 without question. 5.13s are only ~150 rpms more than 4.88s at 70 mph (2922 rpm vs 3071 rpm). I run 35s/5.13s/6 speed and like the set up. I've just adjusted my driving habits to slow down on the interstate. If you regularly drive at 80 mph+, go with 4.88.
 
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I'm headed towards 5.13 with the 6 speed based on feedback here with 35s. The issue of course when I'm now ready to order is revolution is out of stock for hpd30 in 5.13

Look at some vendors that sell them. When they were out of stocks of 5:13s I was able to get a set through poly performance. They shipped very quickly, like within a week, direct from revolution.
 
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Look at some vendors that sell them. When they were out of stocks of 5:13s I was able to get a set through poly performance. They shipped very quickly, like within a week, direct from revolution.

Thanks, I saw they listed some in stock. Ordered so we will see what happens.
 
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If you want to “test drive” 4.88’s on the highway, just use 5th gear instead of 6th. The RPM’s will be almost identical.
I regeared my last 6 speed LJ Rubicon to 4.88’s and ran 35’’ BFG’s and felt it was perfect.
 
Note that the NSG370 is a bit different than the two 5-speed transmissions. It has a noticeably lower overdrive ratio (0.84) than the AX-15 or NV3550 (0.78 or 0.79). As such, the gearing recommendations are a little different as well, and generally about a single step different.

I would not recommend going past 4.88 for a couple reasons:
(1), with 4.88 and 34" nominal tires, you're already running about 3,000 RPM in 6th gear. Going much higher isn't going to net any significance.
(2), beyond 4.88, 4th gear becomes less useful at freeway speeds. With 4.88 and 34s, 4th gear is running about 4,500 RPM at 70 MPH. 4,500 happens to be very close to the peak horsepower RPM, and beyond that RPM, you actually lose power. You can upshift to 5th, but you still won't have as much power as you did in 4th.

This logic does not apply with the 5-speed, and in that case I would actually recommend 5.13. The 5-speeds both have a noticeably taller OD ratio that makes up the difference, and also may only have two gears that are effectively used on the freeway regardless, unlike the NSG370, which has three.

With a 5-speed, think of your gear options on the highway as:
5th gear - cruising gear
4th gear- hill and passing gear

But with a 6-speed, you have an additional option:
6th gear - cruising gear
5th gear - mild hill gear, slow passing gear
4th gear - steep hill and fast passing gear

With 5.13, you'd really start losing the usefulness of 4th in the 6-speed scenario, meaning passing and hill climbing actually becomes noticeably harder at freeway speeds. However, with the 5-speed, 3rd wasn't really an option in the first place, so you might as well plan on using 4th for hills and passing.

That said, I do recommend 4.88 over 4.56 in this scenario. That would be similar in OD performance to a 5-speed and 5.13 gears.
 
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I've run the 6 spd with 33s/4.56 and 35s/5.13.

For 34s, I'm confident you'll be happy with 4.88.
 
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