Hey y’all, I know this thread has been discussed, but it’s time for me to pull the trigger. I have a 97 tj sport that I’m moving from 33’s to 35’s. I’m currently running 3.73 gears and wanting to regear when adding front and rear Eaton e lockers and chrommolly shafts. I’ve read the so you want to run 35’s thread and done everything else. It’s a manual 5 speed I6 and I’m torn between 4.88 and 5.13 gears. It’s mostly off-roading, but some highway driving to get to off-roading in Idaho. Deep snow, mud, and rock crawling trips. I sometimes flat tow to locations. Help would be appreciated. First post and thanks y’all. Leaning towards 4.88.
5.13 or 4.88 gears
- Thread starter Goosse
- Start date
Local shop told me that 4.88 was to low and recommended 4.56, but I wasn’t having that. So I was hesitant to jump to 5.13.
RPMs for a 35" tire. You want 5.13.
AX15/34"/3.73=2184rpm @ 75mph
AX15/34"/4.10=2411rpm @ 75mph
AX15/34"/4.56=2670rpm @ 75mph
AX15/34”/4.88=2857rpm @ 75mph
AX15/34”/5.13=3004rpm @ 75mph
AX15/34"/3.73=2184rpm @ 75mph
AX15/34"/4.10=2411rpm @ 75mph
AX15/34"/4.56=2670rpm @ 75mph
AX15/34”/4.88=2857rpm @ 75mph
AX15/34”/5.13=3004rpm @ 75mph
A shop that doesn't understand how to select an axle gear ratio
What about fuel economy? Doesn’t seem like much difference in rpm on chart’s below, but would 4.88 be better?
You have overdrive. 4.56 is what you use with 35s and an old 4 speed.
Mine was running 4.88 with the 5 speed and 35s up until recently. It isn't quite enough. That was a much nicer ratio for 33s.
I have a 97 with a 4.0 and AX15. I have 4.88s and 35s. I live 7k feet above sea level with mountains.
Unless you live at sea level in flatland, go 5.13s. 4.88 is a huge improvement over stock, but I could definitely use a bit lower gearing at highway speeds in the mountains. It gets lugged down in 5th pulling a hill.
Unless you live at sea level in flatland, go 5.13s. 4.88 is a huge improvement over stock, but I could definitely use a bit lower gearing at highway speeds in the mountains. It gets lugged down in 5th pulling a hill.
I don't buy into the altitude argument for a higher gear ratio. Having driven mine in the low altitude and flat Midwest quite a bit, there was never a time I thought the gearing should be higher.
I don’t mind downshifting on bigger hills, but right now 5th gear is dead to me even on flat ground.
To give you an idea of what to expect, here is something you can test right now.
Given that you currently have an AX-15, which has a 0.79 OD ratio, a 1:1 4th gear, and 3.73 gears, you can get a pretty good feel of what 5th gear would be like by pretending 4th gear is 5th gear on the highway. 3.73 divided by 0.79 is about 4.72, or almost exactly halfway between 4.56 and 4.88.
So take it out on the highway and leave it in 4th gear. Pretend 5th doesn't exist. Get a feel for this performance. 4.56 will have slightly less power and slightly lower RPMs in 5th than you do in 4th now, and 4.88 will have slightly higher power and slightly higher torque in 5th than you do now in 4th. 5.13 would be a small increase in torque, power, and RPM as well.
In your case, I would recommend the 5.13 or at least the 4.88. The 5.13 would put you very close to the OD RPMs of a factory Rubicon with NSG370, so still not fast enough to cause any issues. The only real advantage I could see to 4.88 over 5.13 is that 4.88 leaves 3rd gear a bit more useful at freeway speeds. But with 5.13, 4th gear is already quite powerful at freeway speeds, and 3rd in 4.88 really won't have much,if any, additional power over 4th in 5.13.
Given that you currently have an AX-15, which has a 0.79 OD ratio, a 1:1 4th gear, and 3.73 gears, you can get a pretty good feel of what 5th gear would be like by pretending 4th gear is 5th gear on the highway. 3.73 divided by 0.79 is about 4.72, or almost exactly halfway between 4.56 and 4.88.
So take it out on the highway and leave it in 4th gear. Pretend 5th doesn't exist. Get a feel for this performance. 4.56 will have slightly less power and slightly lower RPMs in 5th than you do in 4th now, and 4.88 will have slightly higher power and slightly higher torque in 5th than you do now in 4th. 5.13 would be a small increase in torque, power, and RPM as well.
In your case, I would recommend the 5.13 or at least the 4.88. The 5.13 would put you very close to the OD RPMs of a factory Rubicon with NSG370, so still not fast enough to cause any issues. The only real advantage I could see to 4.88 over 5.13 is that 4.88 leaves 3rd gear a bit more useful at freeway speeds. But with 5.13, 4th gear is already quite powerful at freeway speeds, and 3rd in 4.88 really won't have much,if any, additional power over 4th in 5.13.
Excellent! Thank you for that. I’m pretty sure I will go 5.13 then this has been helpful
To add more fuel, check out this comparison:
On the left is a bone stock 2006 Rubicon with the NSG370 6-speed manual. The OD ratio in the NSG370 is only 0.84:1 (unlike your 0.79:1). The Rubicon also came with 245/75R16 MTR tires (681 revolutions per mile) and 4.10 axle gearing, as well as the 4:1 transfer case (in place of your 2.72:1 case).
On the right is your proposed 5.13 regear with 35s and AX-15, using 598 revolutions per mile (typical for a 35x12.5R15 MTR) for the tires.
Compare the engine RPMs in the overdrive gear (6th for the left, 5th for the right). They are exceptionally close. So your proposed build would have a final effective gearing very close to something that left the factory. So there is no significant risk of damaging the engine or transmission, nor is it too deep to be drivable.
Also note that 35s and most modifications result in worse aerodynamics, meaning the power demand at a given speed will be higher for a lifted rig on 35s than for a bone stock Rubicon. So you will actually still have less reserve power than the factory Rubicon in overdrive, meaning the deep gearing will be more valuable in your case than it was for the factory Rubicon.
In low range, due to the difference in transfer case ratios, you will still be effectively geared way taller than any Rubicon was.