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Old 04-27-2008, 03:22 PM   #1 (permalink)
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mpg + jeep - thinking cap help?

Hello I stubbled on to this forum by accident (or destiny?) while searching for some information. I was wondering if anyone could help. I have a 2003 Jeep: 4 liter engine, 33" tires, 3.73 gears and the 42rle auto trans in it. The OD gear in it is a stratospheric .69. (third and OD has lock capabilities)

What I need to figure out is what gearing should I run in the axles to achieve the most fuel economy? I understand that I have the aerodynamics of a brick, but right now on the highway the RPM's spin about 1700 rpm @ 65. Anything slower the engine lugs (forget about passing or small inclines on the highway)

options : 4.56 gears - 2058 RPM or 4.88 gears 2228 @ 65. Jeep people swear that 4.88 is better choice (with this transmissions od of .69) but I fear that fuel efficiency is not taken into consideration (this is my daily driver). If I understand fuel economy has many facets one of them is piston speed. An old book written by a guy named Taylor stated that fuel efficiency is maximum when piston speed is at 1000 to 1400 ft/min or 16.4- 19.8 ft/s. That being said, 2000 rpm is the edge of this calculation with the piston moving in the 4L at about 19. But then again Taylor wrote this book soon after WW2 and since then we have synthetic oils and fuel injection - so I dont know if the PS ranges still apply to my 4L engine.

Which one of these gears would be a better choice (40 %city 60% highway)? I am so confused... Right now I get about 17.49 MPG in the highway @ 65-66mph and since regearing is $$$ I dont want regrets with a decrease in fuel economy.

Any thoughts or opinions would be greatly appreciated!

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Old 04-27-2008, 07:15 PM   #2 (permalink)
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Welcome to the site! I'm not a jeep expert, but we do have a few, so I'll just wish you luck and let them chime in with their unique wisdom,
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Old 04-27-2008, 07:26 PM   #3 (permalink)
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4.10-4.11's if you can. I have 4.10's, with 33's, with a .80[or is it .75??] overdrive, and at 65 my rpms are at 2250. Granted I have a ranger, with the 4.0 V6 and a different trans but gear ratios can be set up similar. 4.56's would be nice offroad though. Just figure out where your engine like to be at, and look at a gear chart to figure out what area you can be at
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Old 04-27-2008, 10:33 PM   #4 (permalink)
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I would stay with your stock gearing and drive at 55mph. You are already essentially maximized gearing-wise, since you are pulling the lowest rpm without lugging your engine. Changing your gearing will probably be detrimental, if anything.

Piston speed is still relevant as it is a function of the flame front speed. Charles Taylor was probably talking about the piston speed that maximized mean effective pressure (mep).

I would look at doing some aeromods, especially with 33" tires. Hopefully zjrog will post, as he's been seeing excellent fuel economy for a jeep. Likewise, I would take a look at his thread, located a couple down from this one.

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Old 04-27-2008, 10:38 PM   #5 (permalink)
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TJ or LJ? Is that 17.49 MPG speedo corrected or off by the stock speedo gear? What other equipment are you running? Bumpers, winch, tire type and brand, etc. Whats your road conditions? Hilly? Flat? Mountains? If it was me, I'd run 4.56s. The 4.0 has more than enough power to grunt along at that speed. With your overdrive 4.10s might be a tad too low. If you want to "try" out 4.10s turn off the OD for a while and see how it drives.

For those of us with 5 spds stock @ 65 is around 2200 RPM hence the recommendation of 4.88 for non rubis 42rle and 5.13s for rubis 42rle. If you adhere to the Jerry fan club than you want to run as deep of a set as possible for the greatest passing power and ultimate crawl ratio. IMO I think thats crap for the most part. You don't need that much gear for the 99.5% of the time you aren't off road
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Old 04-28-2008, 09:49 PM   #6 (permalink)
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Drive slower. Drive something else. Whatever you do with your gearing, the gains will be modest to nothing.
Oh, frank lee, I always love how you have something optimistic to say

I don't know about you, but I run about 2000rpm at 70 with stock gearing on my ZJ. I run about 1700 at 60, and at 65, I run about 1850.

Quote:
but right now on the highway the RPM's spin about 1700 rpm @ 65. Anything slower the engine lugs (forget about passing or small inclines on the highway)
Are you saying any slower speed (55mph?) the engine lugs? how much off-roading do you do? because right now, that's your best gearing. at 2000k, my MPG's are real bad... I'm thinking about 16-18mpg. Lately I've been getting about 23-24mpg's doing 55 and driving right, so maybe a slower speed (60 ok?) will net you the best mpg boost with no money or regearing involved.
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Old 04-28-2008, 10:15 PM   #7 (permalink)
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LOL that made my day.

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