Change rears = better fuel mileage?

60 Biscayne · #1 (**permalink**) 01-16-2008, 09:26 AM

I've got an '89 Wrangler that's a 4.2 & 5 speed right now..... I just bought a 2004 Wrangler 4.0 with the automatic trans & transfer case which I'm going to put in as soon as the weather warms up a bit.
Right now my gas mileage stinks & I know the 33x12 Thornbirds don't help.
I was wondering if I changed the rears to a 3.07 ratio vs what I've got now which has to be 4.11 or 4.56 would the effort and expense be worth it? Just had another thought - I wonder what the ratio was of the 2004 Wrangler that this engine came out of & if the computer is set up to recognize that & if I'm going to screw with some shifting ssignals etc......... hmmmmmmmmm...............
Any input would be appreciated,
Thanks - Jim

60 Biscayne · #2 (**permalink**) 01-16-2008, 11:30 AM

Anybody done this kind of swap that can Help...............
Jim

Wrangl3r93 · #3 (**permalink**) 01-16-2008, 11:38 AM

You'll lose considerable power trying to push those big tires and I would think drop your mileage even more by doing so. Although I have no experience with either engine personally. Someone is going to have to chime in and correct me but a stock 4.2 and 5 speed should have somewhere around 3.5? or 3.73 gearing already? I think only the 4bangers came from the factory with the 4.11 rear end.

60 Biscayne · #4 (**permalink**) 01-16-2008, 02:45 PM

Hay thanks,
If your right & the big tire situation is very logical by doing nothing would save me a 600 mile round trip to buy the 3.07 rears plus the purchase price, not to mention the labor to switch them over.
Gas money for my son's Dodge Hemi to go get them was scary plus the tolls........... Boy I feel like I just hit the lottery or something!
I always new that doing nothing would pay off someday!

mud4feet · #5 (**permalink**) 01-16-2008, 02:50 PM

Quote:

Originally Posted by Wrangl3r93

.........Someone is going to have to chime in and correct me but a stock 4.2 and 5 speed should have somewhere around 3.5? or 3.73 gearing already? I think only the 4bangers came from the factory with the 4.11 rear end.

I believe you're correct. Beginning in '03, the Rubi's came with 4.11's, but prior to that, I believe only us 4banger owners were blessed with that gearing. Not sure of the gearing in the 6's, though.......I know it did vary.

Wrangl3r93 · #6 (**permalink**) 01-16-2008, 03:02 PM

Yeah just got back from class so I have more time to search. Depends on the package you got I've seen from 3.07 to 3.73 on the i6's.

Bounty__Hunter · #7 (**permalink**) 01-17-2008, 02:38 AM

An '89 4.2L 5spd will have 3.07 gears from the factory.

If you want to regain power and improve mileage you will install lower (numerically higher) gears to offset the taller heavier tires. 4.11 would be a good swap and they were available in YJ's with the 4cyl 5spd. A lot cheaper and easier to buy the used 4cyl axles and swap them in complete.

Losing the Stuckbirds/Thornterds would help too, they are a very heavy bias-ply tire that sucks onroad and off. They're big with the show-truck crowd because they LOOK aggressive, but the long center tread blocks cause the tire to behave like slicks onroad and off. I've taken my tire groover to a few sets for other jeepers and it has helped 'some'. Get a decent radial MT, it will work much better onroad and off for you.

jps4jeep · #8 (**permalink**) 01-17-2008, 01:47 PM

Quote:

Originally Posted by mud4feet

I believe only us 4banger owners were blessed with that gearing.

and cursed with only a hundred horse.

LBR · #9 (**permalink**) 01-17-2008, 05:07 PM

Quote:

Originally Posted by 60 Biscayne

Hay thanks,
If your right & the big tire situation is very logical by doing nothing would save me a 600 mile round trip to buy the 3.07 rears plus the purchase price, not to mention the labor to switch them over.
Gas money for my son's Dodge Hemi to go get them was scary plus the tolls........... Boy I feel like I just hit the lottery or something!
I always new that doing nothing would pay off someday!

Hi Jim - just curious where you reside that you can't pay a few $$$$ for delivery of a ring & pinion set instead of all the costly $$$$ and driving time horsing around listed above? No UPS or FedEx services available? LBR

redrooster · #10 (**permalink**) 01-18-2008, 10:02 AM

I'm not sure about your computer compatibility with the swap. Install it and see what it does. I'm not up on the newer Jeeps and don't know if they had electronically controlled transmissions, anti-theft system or what. Worst case is that you have to get the computer flashed to rewrite the gearing and remove the anti-theft. Not a big deal.
I would think your mileage would increase slightly with the fuel injection anyhow. Put the engine and all in before you sweat the gears and tires.

Bounty__Hunter · #11 (**permalink**) 01-19-2008, 01:19 AM

Quote:

Originally Posted by jps4jeep

and cursed with only a hundred horse.

More HP than the 4.2L

Megamole · #12 (**permalink**) 01-24-2008, 09:17 PM

Minus the Torq!

Megamole · #13 (**permalink**) 01-24-2008, 09:21 PM

As you can see in my sig I have 89 4.2L with a simaliar problem not as much tire as you but I can feel a difference. When my house gets finsihed and I finally have a warm garage to work in those axles will help out the milage and power of the 4.2L not to mention make 5th gear one I can use below 60-65mph