5.13 Gears, My Thoughts and Real Numbers

[JTenn]

As many of you know, I swapped my complete axle assemblies out of my 21 Willys and installed Rubicon axles with 5.13 gears. I've been running these for a week and have very quickly acclimated to the new feel of all that low end torque. Needless to say I'm very impressed and happy with my decision to install the 5.13's. If anyone is on the fence about such a steep gear, don't be. My daily commute is 100 miles and my mpg has improved about 1-1/2 mpg so far. From 15.5 to 17. I'm still on 35's for now but these numbers should improve when I go to 37's. So here are the actual rpm numbers.
55 mph- 1900 rpm
60 mph- 2100 rpm
65 mph- 2300 rpm
70 mph- 2400 rpm
75 mph- 2600 rpm
Sorry for the crappy pictures but you get the idea.
Before the swap I was generally running close to 2900-3000 rpm in 6th & 7th gear at 75 mph. Hope this helps anyone thinking about changing gears.

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[Jrap]

So after the gear swap, your comparable RPM's dropped and your MPG's went up. I was under the impression the opposite happens? Looking forward to others comments, I have much to learn here . . .

 

[Jeeperjamie]

That's where my RPMs are at on my Max tow JT with 4.10 gears and 37's. My MPGs are slightly better as well. Do they need more than s week to break in or are 35's to small for 5.13 gears. I'm considering going to 4.56 gears in mine since I'm not planning to go larger than 37's.

This a 10 minute commute in mine on I-85 in the Piedmont/Foothills section of NC. I'm about 1-1/2 hrs from the Smokies

 

[JT1]

So after the gear swap, your comparable RPM's dropped and your MPG's went up. I was under the impression the opposite happens? Looking forward to others comments, I have much to learn here . . .

His RPMs dropped because the truck will use 8th gear now.

 

[JT1]

That's where my RPMs are at on my Max tow JT with 4.10 gears and 37's. My MPGs are slightly better as well. Do they need more than s week to break in or are 35's to small for 5.13 gears. I'm considering going to 4.56 gears in mine since I'm not planning to go larger than 37's.

This a 10 minute commute in mine on I-85 in the Piedmont/Foothills section of NC. I'm about 1-1/2 hrs from the Smokies

Jamie, that seems weird. On 4.10s you should be running about 1900rpm in 8th at 75, 2400 in 7th.

 

[Jeeperjamie]

Jamie, that seems weird. On 4.10s you should be running about 1900rpm in 8th at 75, 2400 in 7th.

That's my video I took of mine about a month after I had the Kanati 37 12.5 17's installed. It made be weird but that's still the performance I'm getting out of mine. Depending on how much I sit Idle I still average just over 17.

 

[JT1]

That's my video I took of mine about a month after I had the Kanati 37 12.5 17's installed. It made be weird but that's still the performance I'm getting out of mine. Depending on how much I sit Idle I still average just over 17.

The video is popping an error, so I can't see it.

 

[Jeeperjamie]

Jamie, that seems weird. On 4.10s you should be running about 1900rpm in 8th at 75, 2400 in 7th.

That's about right, I was particularly talking about the mpgs being slightly better, RPMs are what you say. My MPGs are between 17.2 and 17.9 depending on winter vs summer driving and how much I let the truck idle. I'm lifted about 3 inches overall with some weight on it from bumpers, winch, etc.....

I assume the real benefit of the 5.13 gears would come with towing. I'll see if I can get good link to the video

 

[JT1]

That's about right, I was particularly talking about the mpgs being slightly better, RPMs are what you say. My MPGs are between 17.2 and 17.9 depending on winter vs summer driving and how much I let the truck idle. I'm lifted about 3 inches overall with some weight on it from bumpers, winch, etc.....

I assume the real benefit of the 5.13 gears would come with towing. I'll see if I can get good link to the video

The automatic trucks are alot more tolerant of bigger tires, but your mileage is pretty spectacular.

I can squeeze 17mpg out of mine on the highway rolling 65-70. Once I go faster than that, It sits at 14.

 

[Jeeperjamie]

 

[JTenn]

That's where my RPMs are at on my Max tow JT with 4.10 gears and 37's. My MPGs are slightly better as well. Do they need more than s week to break in or are 35's to small for 5.13 gears. I'm considering going to 4.56 gears in mine since I'm not planning to go larger than 37's.

This a 10 minute commute in mine on I-85 in the Piedmont/Foothills section of NC. I'm about 1-1/2 hrs from the Smokies

The 35's are not ideal for the 5.13's but I'm now easily running in 8th gear which is lowering the rpm back close to where it was with 3.73's in 6th and 7th gear. The axles were already built when I bought them. The gears have 5000 miles on them so they're broke in.

 

[JTenn]

So after the gear swap, your comparable RPM's dropped and your MPG's went up. I was under the impression the opposite happens? Looking forward to others comments, I have much to learn here . . .

3.73's wouldn't run in 8th gear. 5.13's easily run in 8th gear. Yeah the rpms dropped but not by some huge amount. Enough to make a slight difference though but the torque gains are definitely noticeable.

 

[Orange01z28]

I'm on 5.13s and 37s and average between 16.9 and 17.9 depending on what crap California-summer blend they're making us use in Phoenix at any given time of the year

I was getting higher numbers with 35s and 3.73s but my truck never used 8th gear and I'm fairly sure the mileage was off because when I first got my Tazer the people who installed my 35s put the tire size as 36- I drove like this for nearly two years

However, the Jeep is just better overall with the 5.13s on and off-road. I can go up the steep highway passes around here like a modern vehicle (ie, without slowing down) and the performance off-road is nothing short of amazing

If you're planning on going to 37s, the minimum gear you should look at is 4.88. The ZF 8 speed is good enough that you can bolt on 37s onto stock 3.73s and still be fine on the road and freeway, but your performance will suffer off-road and you'll be going like 45mph up mountain passes

 

[JTenn]

I'm on 5.13s and 37s and average between 16.9 and 17.9 depending on what crap California-summer blend they're making us use in Phoenix at any given time of the year

I was getting higher numbers with 35s and 3.73s but my truck never used 8th gear and I'm fairly sure the mileage was off because when I first got my Tazer the people who installed my 35s put the tire size as 36- I drove like this for nearly two years

However, the Jeep is just better overall with the 5.13s on and off-road. I can go up the steep highway passes around here like a modern vehicle (ie, without slowing down) and the performance off-road is nothing short of amazing

If you're planning on going to 37s, the minimum gear you should look at is 4.88. The ZF 8 speed is good enough that you can bolt on 37s onto stock 3.73s and still be fine on the road and freeway, but your performance will suffer off-road and you'll be going like 45mph up mountain passes

However, the Jeep is just better overall with the 5.13s on and off-road.

I completely agree with this and that was the point of the post. I'm not saying I'm seeing huge gains, they are marginal but a gain is moving in the right direction. Now on the torque side, yeah thats a huge gain. I was really questioning if 5.13s were going to work for me. I guess it really didn’t matter because I found what I wanted and bought it. That's what the differentials had in them so here we are. It's only going to get better with 37's.

 

[JTenn]

As many of you know, I swapped my complete axle assemblies out of my 21 Willys and installed Rubicon axles with 5.13 gears. I've been running these for a week and have very quickly acclimated to the new feel of all that low end torque. Needless to say I'm very impressed and happy with my decision to install the 5.13's. If anyone is on the fence about such a steep gear, don't be. My daily commute is 100 miles and my mpg has improved about 1-1/2 mpg so far. From 15.5 to 17. I'm still on 35's for now but these numbers should improve when I go to 37's. So here are the actual rpm numbers.
55 mph- 1900 rpm
60 mph- 2100 rpm
65 mph- 2300 rpm
70 mph- 2400 rpm
75 mph- 2600 rpm
Sorry for the crappy pictures but you get the idea.
Before the swap I was generally running close to 2900-3000 rpm in 6th & 7th gear at 75 mph. Hope this helps anyone thinking about changing gears.

@dcmdon

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