5.13 Gears, My Thoughts and Real Numbers

[JTGuy]

After much thought I have scheduled the gear change for. 11-27 to get the Dana Spicer 4.88s. They will be REM polished so no break in or oil change needed. I have the BFG KO2s in 37 C. With my factory 4.10s its not bad with the 8 speed now but MPG is about 14 so I hope it gets a little better. I really like the 37s and will not go bigger. The 4.88s should be a big difference over the 4.10s . My JTR came with 33s and I have not corrected the speedometer and it's right at 10% slow now. Doing the math 4.56 would be closer to factory so the 4.88s will give me just a. little more without going into high revs. I don't care if it goes into 7th on the hills.

Sponsored

 

[AustinL911]

After much thought I have scheduled the gear change for. 11-27 to get the Dana Spicer 4.88s. They will be REM polished so no break in or oil change needed. I have the BFG KO2s in 37 C. With my factory 4.10s its not bad with the 8 speed now but MPG is about 14 so I hope it gets a little better. I really like the 37s and will not go bigger. The 4.88s should be a big difference over the 4.10s . My JTR came with 33s and I have not corrected the speedometer and it's right at 10% slow now. Doing the math 4.56 would be closer to factory so the 4.88s will give me just a. little more without going into high revs. I don't care if it goes into 7th on the hills.

Let me know how the REM polish works out for you as I've been considering doing the same before sending mine off for a transplant.

 

[ttn333]

Let me know how the REM polish works out for you as I've been considering doing the same before sending mine off for a transplant.

Not sure how one would know how well rem polish work unless something goes wrong. I had it done and put about 1200 miles on it immediately (after the initial heating cooling cycpe of course). Also got the fluid changed after that. The mechanic thought he saw some metals but I couldn't see anything cause I didn't have my reading glasses lol. Still running fine so far.

 

[JTGuy]

Every new Gladiator sold is ready to go on a trip. You can buy a new truck and tow with it the next day.

 

[GRIFFBDCO]

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.

So I bought a used gladiator back on 2021.
It is a 2021 Overland. It already had work done to it, 3” lift, 38” tires.

the listing for it had all the original stock specs including the gear ratio. So it did not list the lift in the tires.

but… I was considering switching to 5.13 gears until I compared my RPMs to the ones you posted. Identical.

I now have a 3.5 lift, still 38” tires, avg 17 mpg, identical rpm’s. What’s the odds I am already regraded based on that?

 

[Wheelin98TJ]

So I bought a used gladiator back on 2021.
It is a 2021 Overland. It already had work done to it, 3” lift, 38” tires.

the listing for it had all the original stock specs including the gear ratio. So it did not list the lift in the tires.

but… I was considering switching to 5.13 gears until I compared my RPMs to the ones you posted. Identical.

I now have a 3.5 lift, still 38” tires, avg 17 mpg, identical rpm’s. What’s the odds I am already regraded based on that?

Is your speedometer accurate?

There are a couple other ways you can verify your axle ratio:

1. Count driveshaft revolutions per tire rotation. Raise the rear end and rotate a tire one complete rotation while counting how many times the driveshaft rotates. If the driveshaft rotates just over 5x, you have 5.13. If it rotates just under 4x, you have 3.73. If both tires do not rotate, you have to make two rotations with a single tire.

2. Pop open the diff cover and look for the tooth count stamped on the side of the ring gear.

 

[Stan H]

"If both tires do not rotate, you have to make two rotations with a single tire."

That won't make any difference if both rear tires are off the ground and one rotates forward and the other backwards. That ring is still gonna rotate that pinion.

 

[GRIFFBDCO]

Is your speedometer accurate?

There are a couple other ways you can verify your axle ratio:

1. Count driveshaft revolutions per tire rotation. Raise the rear end and rotate a tire one complete rotation while counting how many times the driveshaft rotates. If the driveshaft rotates just over 5x, you have 5.13. If it rotates just under 4x, you have 3.73. If both tires do not rotate, you have to make two rotations with a single tire.

2. Pop open the diff cover and look for the tooth count stamped on the side of the ring gear.

Thanks I will try this. Speedometer is accurate. A lot of work to this jeep was done before I bought it (only had 3k miles when I got it). Hoping I hit the gear ratio jackpot after I check this.

 

[JTenn]

So I bought a used gladiator back on 2021.
It is a 2021 Overland. It already had work done to it, 3” lift, 38” tires.

the listing for it had all the original stock specs including the gear ratio. So it did not list the lift in the tires.

but… I was considering switching to 5.13 gears until I compared my RPMs to the ones you posted. Identical.

I now have a 3.5 lift, still 38” tires, avg 17 mpg, identical rpm’s. What’s the odds I am already regraded based on that?

Really no way to know unless you do what Wheelin98TJ suggested on that reply. Stan H is correct though, no need to spin the axle twice. An axle revolution is a revolution no matter what. One spins forward and the other spins in reverse. And I'm still loving my 5.13's!! Hope you find more goodies than you expected.

 

[ttn333]

I now have a 3.5 lift, still 38” tires, avg 17 mpg, identical rpm’s.

I would leave it well.enough. I'm on 37s, 2.5" lift and get 12.5mpg with 5.13s.

 

[Splenda]

I'm not calling you a liar. I just don't see how regearing has increased your fuel mileage on the road. I see your RPMs at each speed in 8th gear with 5.13s, and those numbers are pretty close to my RPMs in 6th gear with 4.10s. In 7th gear, my RPMs are lower than your 8th gear RPMs. I definitely don't doubt that the 5.13s are better offroad, but as long as the engine stays in the same RPM range, fuel mileage really should not differ.

 

[Zachanadandy]

Easy experiment if you have 4.10s and want to know what your rpms would be with 5.13s is to manually shift into 7th gear. 7th gear with 4.10s is the same final drive ratio as 5.13s in 8th, therefore the rpms and driving dynamics will be identical.

 

[Wheelin98TJ]

"If both tires do not rotate, you have to make two rotations with a single tire."

That won't make any difference if both rear tires are off the ground and one rotates forward and the other backwards. That ring is still gonna rotate that pinion.

Yes the ring is still going to rotate the pinion, but the spider gears split it 50/50. Experiment with it and you’ll see. Or you can search forum posts (maybe not here) for people saying their driveshaft only rotated 1.5-2x for a single tire rotation.

 

[MarkO]

I'm not calling you a liar. I just don't see how regearing has increased your fuel mileage on the road. I see your RPMs at each speed in 8th gear with 5.13s, and those numbers are pretty close to my RPMs in 6th gear with 4.10s. In 7th gear, my RPMs are lower than your 8th gear RPMs. I definitely don't doubt that the 5.13s are better offroad, but as long as the engine stays in the same RPM range, fuel mileage really should not differ.

Increased efficiency.

 

[Zachanadandy]

Increased efficiency.

If low gearing increased efficiency we would get max efficiency driving around in 4 low. Now if most of your driving is stop and go the increased gearing will help getting you moving in 1st gear, but at speed that's the opposite of what happens. If it worked that way over drive wouldn't exist. And yet as fuel economy requirements increase so so the number of overdrives. Virtually every vehicle has 2 now. The c8 has 4 or 5 overdrive gears. The lowest rpm via the tallest gearing possible to still do the work will always be the most efficient. Granted if you're geared so tall that you don't use the top gears in the transmission ever that's inefficient. But if you gear so low that you're always in the top gear you'll loose efficiency period. Running 5.38s and 38s had the Jeep in 8th gear at pretty much any speed above 40mph. Lost 2mpg on average and 3 on freeway roadtrips. Stock gearing will be the most efficient. 4.10s and 33s is equal to 4.59s and 37s. Bumping that to 4.88s is more than enough to make up for rolling resistance. Driven the same 4.88s will deliver better efficiency than 5.13s... and nether will improve mpg over stock. What usually happens is people drive less aggressively because the Jeep is bigger, taller, and heavier post lift and tires and then pretend physics don't exist and magically making the vehicle harder to move through the wind made it more efficient. It just doesn't work like that. If it did we could just drive around in 4lo and get 30mpg.

Sponsored