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Unread 05-19-2013, 09:52 AM   #1
cowboy24
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BAD CJ mileage!!!!

OK, never posted before, but I really need some suggestions-- I built an 85 cj7 with a 383 stroker, 600 cfm holley, B=M turbo 350, NP206, AMC 20 rear w/ forged axles, but left the stock 2.73 gears, thinking of highway (75mph)mileage and RPM. I'm turning 2200 at 75 (33x12.50 procomps) and the BEST mileage I got was 10.2 WTF!!!! what did I do to get such lousy mileage? this same motor in a 70 elgambino got 18 on the highway (and turned 12's in 1/4 mile) any thoughts or suggestions would be GREATLY appreciated. thanks guys!!!

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Unread 05-19-2013, 09:57 AM   #2
wlo2000
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Well you are driving a Brick aerodynamics-wise..Plus 33's aren't helping. I get about 9 with my LJ with the straight six with 3.73 gears and 35's and mine is a six speed manual..Over 55, the gas mileage dramatically decreases.
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Unread 05-19-2013, 10:07 AM   #3
Jack61
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I'm thinking that the engine is working pretty hard to push the Jeep with that gearing, auto trans and lack of aerodynamics. It is possible that the amount of load on the engine that the secondaries (if vacuum) are opening. Other factors like what rpm range the cam is designed to work in, if the carb is properly tuned (especially the power valve) and if the timing is properly set including total advance can have negative effects on power and mileage.
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Unread 05-19-2013, 10:49 AM   #4
John Strenk
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Quote:
Originally Posted by Jack61 View Post
I'm thinking that the engine is working pretty hard to push the Jeep with that gearing, auto trans and lack of aerodynamics. It is possible that the amount of load on the engine that the secondaries (if vacuum) are opening. Other factors like what rpm range the cam is designed to work in, if the carb is properly tuned (especially the power valve) and if the timing is properly set including total advance can have negative effects on power and mileage.
What he said
plus, what tires? What weight?
All that leads to rolling resistance which sucks the gas right out of your wallet.
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Unread 05-19-2013, 11:17 AM   #5
Nereussailor
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Originally Posted by wlo2000 View Post
well you are driving a brick
x2
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Unread 05-19-2013, 12:24 PM   #6
titomars
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I have a CJ7, The engine is a modified and built 258 with a ported 4.0 HO head and a Holley T/A 470 CFM. It has a T-5 with 4:10's on 33X12.50 GY MTR's. The thing gets around 11 MPG in town and offroad. It gets at best close to 13.5 MPG on the freeway doing a calm 65 MPH.

They are bricks on wheels you can't expect Prius economy from them.

Your axle gears are your problem. That 70 ElCamino did not come with 2:75 gears. It most likely had 3:23's or 3:73 plus your tire diameter was most likely in the 26 to 27 inch range so really there is no comparison. Lugging the engine around will kill mileage. So stepping on the accelerator in very low RPM and you go right into the power valve circuit.
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**Originally Posted by wm69 **
This is the kinda whiny crap I would expect to see in the JK forum. We drive rough riding, leaking, gas guzzling CJ's. Rattle can it and throw it in your Jeep.
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1981 Jeep CJ7 258 4.0 HO head, T-5, D300, D30, D44, SOA
1994 Jeep XJ 4.0 HO, AX-15 (My Beater)
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Unread 05-19-2013, 02:34 PM   #7
Ken4444
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Maybe get a Honda.
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Unread 05-19-2013, 03:32 PM   #8
74Maverick
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Tires hurt, aerodynamics hurt. Compound those at 75 and your screwed. I guarantee if you limit yourself(I wouldnt) to 65. You'll see a 2- mpg difference. And even though you have plenty of hp those gears on 33s will suck down extra fuel at lower speeds. Check a tire/gearing chart for fuel economy gears and you'll see.
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Unread 05-19-2013, 05:24 PM   #9
swatson454
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You live in Elkotraz? Holy moly I spent too much time underground up there.

Anyways, I'll second everything the guys have said. At that speed, I'd personally like to see another 500 rpm. She's a luggin' and the PV is probably open.


Shawn
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Unread 05-19-2013, 06:20 PM   #10
brownbagg
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hook a vacuums gauge so you can see it while driving
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Unread 05-19-2013, 07:57 PM   #11
Turokman123
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Originally Posted by Nereussailor View Post
x2
x3. I'm driving with a 302 and 35's getting about 8 in town. It's in the rules, don't get a Jeep if you want good mileage
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Unread 05-19-2013, 09:59 PM   #12
Jack61
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x3. I'm driving with a 302 and 35's getting about 8 in town. It's in the rules, don't get a Jeep if you want good mileage
That's not true. If cruising conservatively I can get close to 18 mpg, have even gotten close to 20 with the 351w I just replaced.
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Unread 05-19-2013, 10:02 PM   #13
74Maverick
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That's not true. If cruising conservatively I can get close to 18 mpg, have even gotten close to 20 with the 351w I just replaced.
Not saying i dont believe you, it is possible from a 55mph tank long cruise but have you corrected your speedometer/odometer gear.
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Unread 05-20-2013, 04:54 AM   #14
John Strenk
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I get 18 to 20 MPG with mine also on 33's with 3.54 gears with corrected speedometer.

But i'm running AT tires not MT tires.
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Unread 05-20-2013, 06:39 AM   #15
Jack61
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Not saying i dont believe you, it is possible from a 55mph tank long cruise but have you corrected your speedometer/odometer gear.
Yes speedometer is dead on. Cruising speed is 65-70 mph. 35s with 4.56 gears and 5 speed work very well together, the old engine had a decent 325 hp where the new engine is about 430 hp so it doesn't need to work hard at cruising speeds. Vacuum is high, well out of the power valve circuit, has the second to the heaviest spring in the secondary's and timing is nearly fully advanced. Not that I'm trying to get best mileage because it is to fun to drive but I do expect mileage from the new engine to go up some as it gets broke in more.
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