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Unread 12-15-2011, 06:49 AM   #1
ctappan94
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AMC 360 vs 258 MPG on 35"s

Hey guys, what would you consider to be better on mpg, the 258 or the 360. I heard the 258 is a real gas guzzler but its what I have. I could do a swap..... but Say I had either of them fuel injected and basically had them equally set up to be as efficient as they both possibly could be. What would yield better mpg on lets say HI way with 35" tires and "proper" gearing??

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Unread 12-15-2011, 07:52 AM   #2
John Strenk
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If they are both equally 'efficient' the lighter one would give better MPG.

Displacement has nothing to do with with MPG. Weight, rolling resistance, speed are more important than displacement.
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Unread 12-15-2011, 08:56 AM   #3
ctappan94
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dude im saying with all other factors constant.. GENERALLY SPEAKING. what gives better economy? stop bringing the science into it.. its a jeep not a space shuttle. its a rather simple question..
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Unread 12-15-2011, 10:01 AM   #4
jm708
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Posts like this crack me up. If you care what your MPG is your driving the wrong Jeep. I have the 360 on 33 bfg's with the Truck avenger 670 and intake with all MSD and stock 30/20 combo and My MPG is around 5 to 12. I still think your asking the wrong question. Pick the motor you want and tune it to get the best MPG if that is what you want/need.
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Unread 12-15-2011, 11:56 AM   #5
SCJeepr
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I have a 258 with TBI running 35x12.50's with 4:10's and get around 15 MPG. People are amazed how "fast" I can run up and down the road with the set up and a straight 6. I have to very close friends with CJ's. One has a 304 basicly stock motor running 33x12.50 and get roughly 13 piddling around town and stuff. The other has a freaking awesome built 360 running 31x10.50's and gets about 6-7. But it sounds so good!
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Unread 12-15-2011, 12:54 PM   #6
I6CJ7
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Generally speaking with a carburetored engine or even TBI a smaller engine will get better mpg, im getting about 17-18 mpg with my 258 (carburetor) on 33" with 3.54 gears. But it runs rich and needs tuning and therefore I know I could get at least 20mpg or more out of it... jeephammer has a 304 he's tuned getting between 20-26 mpg though.
And generally newer fuel injection is so efficient the 5.3 gm and all LS series engines get better than 20 mpg, another guy on here gets 23mpg from his 5.3 with 35" tires.

Engine Displacement has a lot to do with it, along with a well tuned fuel delivery system whatever it may be, and then of course proper gearing...
But You'll never see a big block chevy or any other big block getting more than maybe 16 mpg... EVER... just too big of an engine to feed without tons of fuel
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Unread 12-15-2011, 10:14 PM   #7
BagusJeep
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EPA figures of the era from the Jeep brochures were:

150ci 22 mpg
258ci 17 mpg
360ci 12 mpg (est from the Waggie as never fitted to a CJ)

If you want to break 20mpg in normal use you need the 4 cylinder, of course it may be a tad slow.

The 258ci was a "lightweight fuel efficient" engine according to the FSM of the time. If you want greater efficiency you can change the cylinder head to a 4.0l HO head, you will liberate more power which will make it more driveable which in turn should yield better fuel consumption for that trip. It will never give the neck snapping pull of the 360 but will be lighter with a better power output per litre.

Jeephammer posts tips from time to time on how to modify and retune the engine and carbs for fuel efficiency so it looks possible to outstrip the 17 mpg on a 258ci.

FI is of course desirable but for economy there is more to it than just bolting on a FI system. I have a V12 6.0l with Zytek engine management and Lucas fuel injection in my Jag which will give 23mpg on the highway, but I am always in a hurry.
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Unread 12-16-2011, 06:30 AM   #8
ctappan94
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The last three posts, thank you guys alot! Finally someone can just give a straight answer
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Unread 12-16-2011, 07:01 AM   #9
Foundrydude
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Quote:
Originally Posted by John Strenk View Post
Displacement has nothing to do with with MPG. Weight, rolling resistance, speed are more important than displacement.

Yes and no, but mostly no. You've obviously never owned a 454 in a light vehicle like a vega or nova. They still get 10-12mpg in a chassis that gets 20mpg with a 200ci straight six or little 140 4 cyl.

Weight only comes into play when you start working a small engine hard. At that point added displacement has less of a mpg penalty, but ultimately displacement is the #1 factor. It's less of a penalty.

However, back to the OP's question, I doubt there's more than 1 or 2 mpg difference on the highway with big tires and bad aerodynamics. The little six will be working hard and your foot will be way down, the 6 will be taking in maximum air per revolution with the throttle open that far. The V8 with a barely open throttle now has the opportunity to achieve similar mileage to the 6. The place the six will trump the v8 is around town when it's not being worked hard and has a lot of idling time.

If you're actually concerned with mileage I'd shoot for a V8 around 300 inches. It's a good balance of torque and power and mileage. AMC purists don't wanna hear it but a ford 289/302 or chevy 283/305 will get better mileage than a 304 due to rotating mass and head design.

Since mileage relates to load on the engine, and a lifted big tire truck has higher load on the engine, don't trust the EPA figures above. Higher load is going to make the 6 and 8 mileage closer together.

There is no straight answer. You may think you got one but you're talking modified vehicles and there's too many variables to get the simple answer you're seeking.

Also, when you hear people talking about getting 6mpg from a 360, understand that their engine has jacked up carburetion and timing. I get 6mpg with a carbureted 454 towing the equivalent of 4 jeeps, going 70mph, turning 3100rpm. There is no way a properly tuned small block should get 6 on the highway unless it's towing a house.

4 out of 5 backyard builders don't know how to set up carburetors or distributors and cost themselves huge mileage numbers.

happy jeepin
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Unread 12-16-2011, 08:33 AM   #10
John Strenk
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Quote:
Originally Posted by Foundrydude View Post
Yes and no, but mostly no. You've obviously never owned a 454 in a light vehicle like a vega or nova. They still get 10-12mpg in a chassis that gets 20mpg with a 200ci straight six or little 140 4 cyl.
...
happy jeepin
Hmm. That's what I said.
454 weights 684 pounds Vega 4 cyl motor weighs 290 pounds.
Didn't I say the lighter motor will get better gas mileage?


Quote:
Weight only comes into play when you start working a small engine hard. At that point added displacement has less of a mpg penalty, but ultimately displacement is the #1 factor. It's less of a penalty.
Weight comes into play any time you accelerate. 0 to 10 or 80 to 90.

But on the whole I'm sure there are plenty of big displacement small block engines that weigh less then the 258 (500#). I think a nice Ford V8 weighs 40# LESS than the 258.

Hey, the 360 is only 40 more pounds. I need to loose more than that myself. Put on an aluminum intake manifold and headers and there probably is no weight penalty.


here is a swap list:
http://www.team.net/sol/tech/engine.html
I think it's for engines available in Europe from the list though.
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Unread 12-16-2011, 09:25 AM   #11
jcal73
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This is not directed at any of the posters in this thread.
Just an observation.
It's funny that MPG even comes up. For goodness sake it's a Jeep
Every time that someone asks about MPG there are some wild answers.
Most of the claims about MPG are made and allot of folks are using their stock speedo gears.
Their speedo is most likely reading inncorrectly.

Good stuff!


Jeff
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Unread 12-16-2011, 09:27 AM   #12
jm708
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Quote:
There is no straight answer. You may think you got one but you're talking modified vehicles and there's too many variables to get the simple answer you're seeking
I gave you what MPG I get but agree with foundrydude. Sorry if you dont like it but its true. Where you live, tires, gears, how you drive and more all come into play. Do you have to have good MPG? If so look into the diesel swaps.
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Unread 12-16-2011, 01:00 PM   #13
WoodyHD
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I think VEX gets about 35 aeronautical miles per gallon with his 258 running shuttle fuel.
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Unread 12-16-2011, 01:04 PM   #14
jcal73
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Quote:
Originally Posted by WoodyHD View Post
I think VEX gets about 35 aeronautical miles per gallon with his 258 running shuttle fuel.
You forgot that he uses a hydrogen generator as well!
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Unread 12-16-2011, 02:13 PM   #15
Submariner
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ok, speaking for experience with only 33" tires.

304, 2 barrel with 3 speed, 3:73 gears got 15 MPG, no more no less

258, 4 barrel, offenhouser manifold, headers duel exhaust, 5 sp, 4:10's got 18 mpg

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