JPop

What will rob your fuel economy more than anything is dropping out of OD under load, which also unlocks the converter causing an additional rpm flash and possibly a second downshift.

So here are 3 scenarios with 35s and automatic transmission,

4.88s, maintaining 70mph, aprox 2300 rpm in OD, about 80 rwhp. Under load, converter unlocks, downshift to third, rpms flash to 3700, about 130 rwhp. Under even heavier load, converter still unlocked, vehicle slowing downshift to second, rpms flash to 5000, maxed out at about 170 rwhp.

5.13s, maintaining 70mph, aprox 2450 rpm in OD, about 85 rwhp. Under load, converter unlocks, downshift to third, rpms flash to 3850, about 145 rwhp. Under even heavier load the transmission isn't likely to further downshift as the PCM (powertrain control module) will deem that as an unsafe rpm range. Slower speeds and you are going to get back to that 5000 rpm flash

5.38s, maintaining 70mph, aprox 2600 rpm in OD, about 90 rwhp. Under load, converter unlocks, downshift to third, rpms flash to 4000, about 150 rwhp. At that point you are within 10% or so of the 3.8s max power output, won't be downshifting and you just push the go pedal.

With each step up to a steeper gear you are also reducing the amount of occurrences that you will get the somewhat unnerving downshift and rpm flash. That will make your driving experience more enjoyable and should a time come for the higher revving Pentastar motor in your new 2011/?? JK you are already all over it to do a swap.

chilidipper

Lighter tires without sacrificing durability is the ticket. What tire are you running?

m1ke07jk

Lighter tires??

Skippman

Lighter tires equals less rotational mass and lower unsprung weight.

Unsprung weight is a suspension term used to refer to the wheels effect on the coils and shocks. Think of it like holding out your arm and having a apple in it. You can hold that arm out pretty easily and wave it up and own with no real effort. Now try that with a grape fruit. Then a watermelon. The heavier the load the harder it is to move it or hold it. So thus the heavier your tire/wheel combination the more strain it puts on the axel and suspension. That's why SCCA and Autocross cars run such light wheels.

Rotational mass refers to the overall weight of the tire and wheel. The heavier the tire and wheel, the more rotational mass. The more rotational mass, the harder it is to turn and brake the wheels. Off road tires are notoriously heavy which puts a greater strain on the entire power train and requires more fuel to move. By moving to lighter tires and wheels you reduce this strain. The downside is many of the lighter tires are lighter because they're thinner and are more likely to get punctured.



Back to JPops Post:

If I understand you're logic then the steeper the gearing, the better. With my automatic's overdrive at .69 the steeper gearing is going to put me into the more desirable area of the power band and providing the greatest RWHP. Since I'm running the Dana 44's front and read I can accomidate the 5.38's. This will eliminate most of the sluggishness from the lower RPM band of the engine by keeping it at a higher RPM making acceleration almost immediate rather than having to wait for the engine to wind up, if you pardon the vernacular.

m1ke07jk

thx... I was just thinking he meant lighter as in smaller is all.

lbchip

All these stats are really great, but in all honesty without a Hemo swap you are just polishing a turd

JPop

Those stats should help people manage the expectations of their, as you lovingly referred to it, "turd".

stlfan06

Okay so I have a 08 2dr X. Thinking of putting on 35's down the road. Also looking at 5.13's, but I hear these people that have that setup running like 3k rpms at 70mph. Right now I can run 80mph at about 2400 rpms. I think 70 is like 2100. I don't want to constantly run 3k rpms on the highway.

If I ran 4.88s, would that help out my situation more so than 5.13's?

Skippman

Hmmmm.... $1500 for a gear swap (labor and parts) or $10K+ for a power plant swap. Regearing is in the realm of possiblity for all but the most frugal of JK owners and can be done in a day, maybe two if the shops behind. An engine swap on the other hand will require an expert mechanics full attention for at least a week. That's if you can find a shop in your area willing to take on such an ambitious project. I fail to see how the two are even remotely comperable.



If you run 4.88's it will put you nears as makes no diffrence back to the factory powerband on 35" tires.

l2chil

After regearing I agree with those on this board that say this is the best modification you can do. I would go with the lowest gear size possible.