I'm going to make a decision on some gearing soon. I have an auto running 35's and 373 gearing. I lost 3 mpg after the lift and the 35's. I'm definitely going to re-gear but it's a tough decision between a 488 vs. 513. Pretty flat here in the mid-west, but the jeep is a daily driver and four wheeled as a hobby on the weekend.

I've read where some have tried 513's but were not real happy with that gearing and 35's, thinking the 513 gearing may be better for 37's or so...

I've been trying to make heads or tails out of what may be best for an auto tranny running 35's for all around performance as a weekend warrior and a daily driver.

Getting ready to place an order soon, I was hoping for some additional input on a right choice to make. The difference between the gearing would just be a few hundred rpm, but I really don't know if the 3.8 motor likes to torque or if it likes a higher rpm to reach its sweet spot. Any info or first hand knowledge would be appreciated.

 

Well, the engine likes to run at around 1,800 - 2,000 rpm.

If you are geared to run at YOUR preferred cruising speed when you are in that range, you'll get good mpg.

If you gear too low, so your rpm at your cruising speed is too high, you'll use MORE gas, not less.

btw - losing 3 mpg from 35's is unusual...its only about 8.6% taller than the stock 32's.

Did you recalibrate the speedometer/odometer?

Or are you driving at ~ 71 mph when the speedometer says 65 mph, etc?

Are you adding about 9% to the odometer reading when you fill up, to see how many miles you REALLY went on those gallons?

If you don't, well, you will at least THINK your mpg dropped by that.

 

Have 35s and went with 5.13s. I almost think it could use deeper gears. Im running 2500 RPM at 70 MPH. Better than it was and no more turning off the O/D!

 

As TEEJ mentioned, the sweet spot for efficiency/mpg is around 1800-2000. The Torque curve starts flattening out around 2500 rpm. Flatland highway cruises at 70 mph would make the 4.56s right if they were available and in their absence I would run with the 4.88s.

 

By looking at the chart on the hypertech website that shows torque curve, I would think you would want to keep your R's in the 2000-2500 range.

http://www.hypertech.com/media/dynoc...fica,%2020.pdf

4.88s in OD would yield about 2100 rpm at 65

5.13s would give you about 2200 rpm at 65

Check out the chart on the Hypertech page and you will see for yourself where the motor is the strongest. That should help you to make your decision.

4lo.com also has some good calculators and ideal ratio charts that might prove helpful. BTW, the ratio on the OD for the auto is 0.69. (woohoo)

This motor is quite anemic and personally, I wouldn't want to run below 2K rpm. Keep in mind the added resistance and weight of those 35's.

 

Not really, there is a difference between what you are looking at, and the application in question...IE: Best fuel economy....its better at 1,800 - 2,000 rpm....NOT at higher rpms, where it DROPS.



Remember, most of the "Charts" use a 1:1 final drive ratio, whereas as you pointed out, the automagic trannies have an OD of 0.69...and, in addition to that, the 6 speeds have a final top gear drive ratio of 0.84, etc.

Some of the charts also give a range of pewer to economy...and the rpm for economy is lower than the rpm for power.

For best mpg...you are looking for the power that allows the jeep to cruise at the desired speed...NOT the maximum torque or horsepressure or whatever...

...Think more along the lines of "Just Enough" gumption to move the damn thing...more than that, and you are blowing $ out the exhaust.

So, sure, downshifting adds acceleration/power...and, swapping in lower gears is an expensive way of downshifting....as it ALSO raises your rpm in EVERY GEAR, including when trying to cruise along with as little gas as possible (NOT accelerating...).



So, do I like lower gears?, hell yeah!...but, if the question is about MPG....welll, its not the answer.



This is WHY people use OVERDRIVE...(TALLER GEARING) to get better mpg...the lower the rpms, the better the mpg...until the rpms are too low to make enough power to push the rig through the air.

Swapping in 4.88's for example instead of 3.73's would raise your rpm by about 23% across the board (Every gear).

0.69 + 23% = ~ 0.84...~ the same as the stock 6 speed's final drive ratio.

Now, why did they make the auto have a taller final drive than the 6 spd?



Well, to drop the RPM, and SAVE GAS.

Why is 3.21 the STANDARD diff gearing used by jeep?

Well, its to drop the RPM and SAVE GAS.



The 3.21's drop the rpm by about 14% compared to the optional 3.73's.

I cruise at ~ 2,000 rpm in 6th @ ~ 60 mph, with 3.73's.

If I had the standard 3.21's, I 'd be cruising at more like 1,720 rpm at 60 mph...closer to the maximum fuel efficiency range.

So - according to Jeep, the point of diminishing return between power required to push the jeep, and the fuel used to do it, converges at about 1,720 rpm at 60 mph...more rpm = less mpg.

Raising my rpm to 2,000 at 60 mph gets me worse mpg...but, better power to tow, etc.

The towing limit is about 3,500 lb...and 3.73's were calculated to compensate for the force required to move the extra weight, etc...at a sacrifice in mpg.

and so forth....

Hope that helps a bit!

 

The 3 mpg I'm showing less with the 35's is on the display. The dealership who installed the lift said they would tweak the odometer but either did not do it or didn't get it close enough. It's about 4 to 5 mph off. It's as you stated, when I'm running 70 its reading 65 on the GPS. I never thought about the MPG being off also.

The big concern is the OD kicking in and out. I find myself turning the OD off at cruising speeds on the Interstate. If I just run 55 on a highway the OD seems to be ok, but if I get up to 70 or so the OD is not happy at that range and kicks in and out, I figured due to the extra peddle needed to pull the tires at that speed.

Basically I'm wanting the OD back, I'm afraid the in and out will wear on the drive train, it can't be good for it.


Making a decision for gearing in tending to be a pain, I've read a lot of threads but none yet that's made my mind up. I'll check out some charts, maybe that will help.

 

LOL

Yeah, the OD is trying to decipher the output from the tail shaft saying you are at one speed, and therefore a corresponding torque requirement - and the sensors saying, hey, its not enough ooomph here, disengage and let the non-OD push, etc.



If you recalibrate the speedometer/odometer (Hypertech, etc...), that will go away, clearing the tiny electronic brain's confusion, etc.

The stealerships can only recalibrate to a 32" tire (The largest STOCK size...).

You need a re-programmer to compensate for taller rubber.

J ust
E mpty
E very
P ocket

 

All you guys and your chart talk make me laugh
Unless you have experience driving a Jeep with 35's or 37's with an automatic, and then switching to lower gears, you dont have clue what its really like and need to keep your opinions to yourselves.

I dont give a crap what charts say, because they dont factor in the aerodynamics of a brick or the extra weight of the bigger tires.
This Jeep will not get better MPG's running 1800-2000 RPM on the highway while spinning 35's!
It does not have the power and torque to maintain speeds above 55mph with 35's at 1800-2000 RPM....unless your going down hill with a tailwind
And because of this lack of power and torque, the automatics with 35's running 2k RPM at highway speeds, will spend more time down shifting out of OD then staying in it. When the trans shifts out of OD, the RPMs are much higher, which is not good for MPG at all, not to mention all the wear and tear on the trans while doing so. At least with 4.10's and 35's, the RPM's were much higher with the OD off, compared to running 5.13's and OD on.

With 5.13's, 35's and an automatic, Im getting better MPG all around, then I did with 4.10's and 35's, mainly because the Jeep doesnt have to shift out of OD all the time on the highway, and it in the city it doesnt work as hard to get up to speed, which gives you better MPG too.

So all you chart talkers need to quit talking, at least until you have real hands on experience driving both geared and non geared Jeeps with 35+ tires.....because with 35's on the highway doing 55+ MPH @ 1800-2000 RPM is NOT going to give you better MPG

 

x2....you beat me to it