4:88 or 5:13 for RUBICON WITH AUTO & 35'S

YOGI1 · 09-13-2009, 08:20 PM

I just recieved my new 12:50x35x15 BFG KM2's and MT's 15x8 sidebiter's.I have a 06 rubicon with a auto trans can anyone chime in that are running 4:88's and 5:13's and help me decide on a ratio. this is not my daily driver but i still want to be able to go where I want including the interstate. I don't mind driving a little slower I was leaning towards the 5:13's to be alittle easier on the drive train I also will be installing rear cm shafts at the same time.

**Imped** · 09-13-2009, 08:24 PM

You have a 42rle trans, in case you didn't know. 5.13 for sure. That will help overcome your tall OD.

RJV · 09-13-2009, 08:34 PM

I have 4.88s and 33s. 4.88 was the tallest I could go with because of my front axle. the jeep drives good but wish the gear was a little taller. definetly go with 5.13s if your going to run 35s.

Dmcastino · 09-13-2009, 08:35 PM

5.13 ten characters

**Imped** · 09-13-2009, 08:43 PM

Quote:

Originally Posted by RJV

I have 4.88s and 33s. 4.88 was the tallest I could go with because of my front axle. the jeep drives good but wish the gear was a little taller. definetly go with 5.13s if your going to run 35s.

You're confusing the terms tall with low. Numerically higher gears are "lower" and "shorter", not taller. Taller gears assumes numerically lower gears, which are higher.

3.07 with 35's=very tall/high
5.13 with 35's=short/low

uvalax236 · 09-13-2009, 09:17 PM

I have 5.13's with my 315/75/16 (35's) Nitto Terra Grapplers. Love the gearing!

**mudb8** · 09-13-2009, 11:04 PM

at least 5.13's.... the only thing stopping you from what you need is the rubi locker carrier. that's as low as it gets....

I've had both 4.88's and 5.13's.. my next gear change for the 35/36" tires is 5.89's and bye bye rubi lockers, even tho I love em so much...

pielet97 · 09-14-2009, 06:37 AM

Quote:

Originally Posted by YOGI1

I just recieved my new 12:50x35x15 BFG KM2's and MT's 15x8 sidebiter's.I have a 06 rubicon with a auto trans can anyone chime in that are running 4:88's and 5:13's and help me decide on a ratio. this is not my daily driver but i still want to be able to go where I want including the interstate. I don't mind driving a little slower I was leaning towards the 5:13's to be alittle easier on the drive train I also will be installing rear cm shafts at the same time.

Looks like I"m in the minority, but I have 35 MTRs w/ the 42RLE and 4.88s. I love the way it drives. Its peppy and cruises at 75mph at about 2300 rpm. I would think highway mileage would suck with 5.13s, but I don't know. I get between 15 and 18 depending on the conditions.

robncar · 09-14-2009, 06:53 AM

Quote:

Originally Posted by pielet97

Looks like I"m in the minority, but I have 35 MTRs w/ the 42RLE and 4.88s. I love the way it drives. Its peppy and cruises at 75mph at about 2300 rpm. I would think highway mileage would suck with 5.13s, but I don't know. I get between 15 and 18 depending on the conditions.

You get between 15 and 18MPG with 35's on an LJ!!?? What speedo gear are you running, or if a Rubi, how'd you convert the speedo signal?

The difference between 4.88 and 5.13 is about 5%. So, 1000 RPM becomes 1050, 2300 becomes 2415. It'll hardly kill your highway mileage.

If we had it to do over again, we would have replaced the stock D30 and be running lower than 4.88 with our 35's and 42RLE. This transmission loves low gears.

**bfewkwa** · 09-14-2009, 08:43 AM

I've got 35's and 4.88s with the 42RLE. Just got my Rubi about 3 weeks ago.

After driving it for the first week, my initial impression was that I would want to go with 5.13s in the very near future. After driving with it for a few more weeks and learning the Rubi a little better, I think the 4.88s are good enough for now.

That said, if I were in your shoes and choosing one or the other, 5.13s would probably be my choice.

Here's a thread I had started about the same topic:
http://www.jeepforum.com/forum/f9/35...-gears-845621/

00jeepred · 09-14-2009, 08:47 AM

i know a kid with a rubicon who has 5.13's and 37's with a manual trans. and he goes through r&p gears like water. In the process of going to d-60's. at that low of a gear the pinion is so small that there is not alot of tooth contact.

**Imped** · 09-14-2009, 08:53 AM

Quote:

Originally Posted by 00jeepred

i know a kid with a rubicon who has 5.13's and 37's with a manual trans. and he goes through r&p gears like water. In the process of going to d-60's. at that low of a gear the pinion is so small that there is not alot of tooth contact.

You are very right. OP, unless you'll really be beating the absolute crap out of this rig offroad, then don't worry about the tooth contact issue. Otherwise, you'll love the 5.13's.

**RubiconRazorbac** · 09-14-2009, 09:13 AM

Quote:

Originally Posted by 00jeepred

i know a kid with a rubicon who has 5.13's and 37's with a manual trans. and he goes through r&p gears like water. In the process of going to d-60's. at that low of a gear the pinion is so small that there is not alot of tooth contact.

I can tell you right away, at least in this case, the relative weakness of the R&P is minimal compared to the inherent problems with the driver.

pielet97 · 09-14-2009, 01:07 PM

Quote:

Originally Posted by robncar

You get between 15 and 18MPG with 35's on an LJ!!?? What speedo gear are you running, or if a Rubi, how'd you convert the speedo signal?

The difference between 4.88 and 5.13 is about 5%. So, 1000 RPM becomes 1050, 2300 becomes 2415. It'll hardly kill your highway mileage.

If we had it to do over again, we would have replaced the stock D30 and be running lower than 4.88 with our 35's and 42RLE. This transmission loves low gears.

This was the first trip from Cali to Indiana, I just bought it. I compared the trip odometer to the fuel used and only got into the 14's twice, in the mountains. It is a Rubicon, and I don't think there is a conversion box. I did notice the the speed was indicating about 5mph faster than the GPS, at 75mph(speedo).

We agree the RPMs won't change with gearing, so you're saying the speed is slower and the mileage is less than shown? I'd agree to that. After doing some math, and assuming the 5mph at 70 is exact, my mpg would go to 14-17. About 6.6% less, or 1mpg.

With your statement, only a 5% change in rpm, you're assuming the fuel flow curve is linear, and we all know it is not. You don't burn twice the fuel at 2000 as you do at 1000, and 2K-4K. Just not true, it burns significantly more as the rpms increase.

**RubiconRazorbac** · 09-14-2009, 01:45 PM

Quote:

Originally Posted by pielet97

With your statement, only a 5% change in rpm, you're assuming the fuel flow curve is linear, and we all know it is not. You don't burn twice the fuel at 2000 as you do at 1000, and 2K-4K. Just not true, it burns significantly more as the rpms increase.

That's what you'd think intuitively. However, the Jeep is like a brick-shaped parachute going down the road. You need the gearing to keep you in the power curve for maximum efficiency. You will get better gas mileage at 2500 rpm in the correct gear than you will "lugging" the engine at 2000 rpm in the wrong gear.

With 5.13s it'll take less effort for your engine to keep the Jeep moving even if the rpm's are higher. It's all about being able to overcome the resistance.