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Gas Milage for the TJ


 
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V7Sport73
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Joined: 01 Nov 2006
Location: El Paso, Texas

PostPosted: Sat Dec 23, 2006 9:04 pm    Post subject: Gas Milage for the TJ Reply with quote

I know this is a subject of much controversy but I would like to know what other people are getting for gas mileage out of there TJs. I have a 2006 Rubicon with 35 inch Goodyear MT/R tires and 411 axles. I have about 500 pounds of after market gear bolted on the Jeep. I installed a Truspeed speed calibration/correction unit. Indicated MPH on my speedo matches my GPS. I am currently getting about 14 MPG in the city. Is this about average? Will changing my gears to 488 help or make it worse? I originally was getting about 15.5 MPG prior to changing tires and bolting on all those overweight goodies. I track my mileage religiously. Just want to know about other peoples experience with this subject.
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ehirner
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Joined: 01 Jan 2004
Location: Eastern PA

PostPosted: Sat Dec 23, 2006 9:23 pm    Post subject: Reply with quote

That's normal. Your mileage will probably stay the same with the new gears. The weight, poor aerodynamics, and inefficient engine outweigh any benefits with regards to better mileage from regearing.
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TJ Apex
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Joined: 29 Jul 2005
Location: Mountains of SoCal. 6257 elev.

PostPosted: Sun Dec 24, 2006 9:47 am    Post subject: Reply with quote

I know I've said this many times, but I average about 18 and have gotten as much 20. But my rig is stock except for 32" tires. And I do alot of freeway driving, the only bad thing is I go from 6000+ down to 900ft elevation and back up everyday.
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Lowrider
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Joined: 08 Nov 2005
Location: Washington State

PostPosted: Sun Dec 24, 2006 10:33 am    Post subject: Reply with quote

My bone-stock '06 Unlimited gets about 15-16mpg around town and on a 3500-mile trip last Spring, mostly on 2-lane backroads, I got as much as 23mpg. The funny thing is, I can get 21+mpg running on level roads at 65mph, but if I kick it up to 75mph my mileage drops off to 19mpg or so. The difference between 65 and 75 is only about 100-150rpm and at 75mph the engine is only turning 2250rpm - right where I'd expect the engine to be its most efficient.

I believe the reduced mileage is a function of the poor aerodynamics of these vehicles and I believe the aerodynamic drag isn't linear but increases on a curve with the road speed. I'm just speculating here but it's the only answer I can come up with in my feeble mind. Mr. Green

For example: If the amount of horsepower needed to push the air aside at 60mph is X, the horsepower needed to push through the same air at 75mph might be 2X, instead of the 1.25X you'd expect if it was a linear increase.

Does this make sense to anyone else or am I having an acid flashback? Mr. Green Mr. Green
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2006 Wrangler Unlimited
1997 Cherokee Sport 2WD
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TJ Apex
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Joined: 29 Jul 2005
Location: Mountains of SoCal. 6257 elev.

PostPosted: Sun Dec 24, 2006 11:06 am    Post subject: Reply with quote

I get it perfectly...The amount of energy needed to make the power for just a few more MPH is almost exponential.
As an example, land speed vehicle designers spend a lot of time on aerodynamics first, and then make the horsepower simply because horsepower is easer to gain (with enough cash).
Aerodynamics, I agree is the biggest killer of gas mileage for “true Jeeps”. Cherokee's with 4.0 tend to get more MPG with a 4.0 just because of the angle of the windsheild.
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Why do I have to press one for English?!
Welfare recipients should pass a drug test to get paid…

02' Apex Edition,
2" BB, 31" BFG AT's
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ehirner
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Joined: 01 Jan 2004
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PostPosted: Sun Dec 24, 2006 11:42 am    Post subject: Reply with quote

Lowrider wrote:
My bone-stock '06 Unlimited gets about 15-16mpg around town and on a 3500-mile trip last Spring, mostly on 2-lane backroads, I got as much as 23mpg. The funny thing is, I can get 21+mpg running on level roads at 65mph, but if I kick it up to 75mph my mileage drops off to 19mpg or so. The difference between 65 and 75 is only about 100-150rpm and at 75mph the engine is only turning 2250rpm - right where I'd expect the engine to be its most efficient.

I believe the reduced mileage is a function of the poor aerodynamics of these vehicles and I believe the aerodynamic drag isn't linear but increases on a curve with the road speed. I'm just speculating here but it's the only answer I can come up with in my feeble mind. Mr. Green

For example: If the amount of horsepower needed to push the air aside at 60mph is X, the horsepower needed to push through the same air at 75mph might be 2X, instead of the 1.25X you'd expect if it was a linear increase.

Does this make sense to anyone else or am I having an acid flashback? Mr. Green Mr. Green

Correct, it's not linear. It's exponential.
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V7Sport73
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Joined: 01 Nov 2006
Location: El Paso, Texas

PostPosted: Mon Dec 25, 2006 5:04 pm    Post subject: TJ Gas mileage Reply with quote

I forgot to mention that I have the Auto transmission in my Rubicon. I'm thinking this contributes to the poor gas mileage.
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Lowrider
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Joined: 08 Nov 2005
Location: Washington State

PostPosted: Mon Dec 25, 2006 10:35 pm    Post subject: Reply with quote

My Unlimited is ALSO an automatic.
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1997 Cherokee Sport 2WD
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TJ Apex
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Joined: 29 Jul 2005
Location: Mountains of SoCal. 6257 elev.

PostPosted: Tue Dec 26, 2006 8:22 am    Post subject: Reply with quote

Automatics do contribute to poor gas mileage, although they are far more efficient that autos from the early 80's and previous, you still have a "load" on the engine when your sitting at a stoplight idling in gear.
Another thing is, autos tend to have wider gear ratio’s, witch adds load on the engine when accelerating.
But aerodynamics as mentioned before, is the biggest killer of MPG.
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Welfare recipients should pass a drug test to get paid…

02' Apex Edition,
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V7Sport73
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Joined: 01 Nov 2006
Location: El Paso, Texas

PostPosted: Tue Dec 26, 2006 4:48 pm    Post subject: aerodynamics and MPG's Reply with quote

I hear you on the aerodynamics dynamics on the Jeep. But I rarely drive above 65 MPH. I am thinking that the engine is really lugging around. At 70 MPH my RPM’s are at 2000. At 65 MPH they are around 1850. The 411’s gears on the Jeep now are tolerable. The Jeep seems to run decent. However at highway speeds is when my lack of power shows up the most. The Jeep downshifts on any kind of hill. I don’t think I can even get it to run 80 MPH if I wanted it to. I was hoping that the lower engine RPM the more economical on gas but that doesn’t seem to be the case. It would be nice to see 15.5 MPG again. For now I will try to live with 14 MPG’s.
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ehirner
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PostPosted: Tue Dec 26, 2006 5:26 pm    Post subject: Re: aerodynamics and MPG's Reply with quote

V7Sport73 wrote:
I hear you on the aerodynamics dynamics on the Jeep. But I rarely drive above 65 MPH. I am thinking that the engine is really lugging around. At 70 MPH my RPM’s are at 2000. At 65 MPH they are around 1850. The 411’s gears on the Jeep now are tolerable. The Jeep seems to run decent. However at highway speeds is when my lack of power shows up the most. The Jeep downshifts on any kind of hill. I don’t think I can even get it to run 80 MPH if I wanted it to. I was hoping that the lower engine RPM the more economical on gas but that doesn’t seem to be the case. It would be nice to see 15.5 MPG again. For now I will try to live with 14 MPG’s.

The overdrive on the 4-speed auto is fairly tall. 4.10s witht he 4-speed auto and 35s is NOT a desireable setup. If you turn off overdrive using the O/D Off switch on the dash, you'll see much better performance and likely a bit better mileage also as you'll be using the engine in a better range of its powerband. I would suggest looking at a regear somewhere down the line to 4.88s (or 5.13s since you've got a Rubicon).
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V7Sport73
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Joined: 01 Nov 2006
Location: El Paso, Texas

PostPosted: Wed Dec 27, 2006 5:49 pm    Post subject: Re-gear Reply with quote

Well I'm thinking very much about the 488's. But since I've never been inside an axle before I more than likely take it to the shop to have it done. I'm still thinking about the $1,450.00 quote I got for the gear swap. Seems like a lot of money. The shop I use here in El Paso seems like a good honest group of guys. He told me the labor cost on each axle is about $350.00. At this point I'm still trying to recover from x-mas. So I don’t plan on doing any gear swapping right away. On the other hand I'm thinking about doing it myself. I will need to learn sooner or later how to repair those axles. Any thoughts or opinion on this?
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ehirner
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PostPosted: Wed Dec 27, 2006 8:19 pm    Post subject: Reply with quote

It's not impossible for a first-timer but it takes awhile. If you don't have another vehicle to DD while doing the gear setup yourself, I'd take it to a pro to have it done.
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TJ Apex
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Joined: 29 Jul 2005
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PostPosted: Thu Dec 28, 2006 8:14 am    Post subject: Reply with quote

That quote sounds about right. For a gear change, you'll need some special tools for the gear swap witch might get costly, plus you'll need when to shim (or not to shim) your pinion etc. If you do it wrong you could have some potentionaly very expensive problems not far down the road.
I'd have a pro do it for you, maybe he or she will show you how it's done so you can perform it later.
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Welfare recipients should pass a drug test to get paid…

02' Apex Edition,
2" BB, 31" BFG AT's
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