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Old 02-08-2006, 05:06 AM   #1 (permalink)
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258 I6 gas mileage

I'm not exactly a Jeep newbie, but figured I'd play it safe and ask the Q? here.

I had a '77 CJ7, (304, T150 3spd, 2.5" spring lift, 1.5" shackle lift, Moser axles, 32" Mud Rovers). The Jeep got about 11 MPG.

I learned a lot owning it of what I needed and didn't. I had to sell it because I had a kid, and it needed some steering work to be safe enough to drive around with a 16month old baby.

So, not my son is 8yrs old, and I'm looking at CJ7's again. This time I'm wanting it with a 258 I6, T176 4speed.

If I keep it generally stock, with 31's. What kind of mileage can I expect?

Back with my first CJ, gas was around 1.25/gal, so the poor mileage wasn't much of an issue. But with current prices, I have to consider MPG before I get one.

This will be my main transportation.

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Old 02-08-2006, 05:57 AM   #2 (permalink)
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Without F.I. they just are not going to pull good milage. My 98 grand cherokee on 31 bfg At doing a constant 55 will get around 20mpg(Best case). Any stop and go or driving above 60 14.5-15(Avarage). This is not somthing you get into because the fuel economy is good. People love their Jeeps so they just don't care
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Old 02-08-2006, 06:57 AM   #3 (permalink)
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I could deal with 15mpg, but if I'll be getting 11mpg with an I6, then I might as well go with a 304 again.

I know I won't get good mileage, but it will be a factor in what I get.

My 86 4Runner only gets 17 with a 4 banger, so I don't expect any better than that.
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Old 02-08-2006, 07:21 AM   #4 (permalink)
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I have a 99TJ 6cyl Automatic, stock with 31's that gets around 200 per tank. I'm lucky is I get 220. So on the high side about 12 mpg.
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Old 02-08-2006, 07:25 AM   #5 (permalink)
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A properly tuned 258 can get very good gas milage. A good ignition system and properly working carburator are the two main things to address. After that drive like you want to save gas and you should be pleased with your MPG.

I know several people that were very happy with Jacobs ignition parts on their CJ's, noticably improved power and MPG. Two of the guys sould the CJ's for YJ's to get rust free jeeps with EFI and were dissapointed with the loss in power AND fuel milage, though EFI is definatly better in the rocks. The basic setup of all the jeeps in question was almost exactly the same
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Old 02-08-2006, 07:40 AM   #6 (permalink)
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258 with weber and 904 automatic, in an eagle wagon gave me 19 all the time... i have had 360 wag that got 17, and 360 J4000 that also gave 17 (both ahd skinny tires) carb vs fi doesnt have anything to do with mileage IF the carb is right... it very much does make a difference on angles, especially in the rocks... any given engine design requires a certain weight of fuel per horsepower per hour, simple fact. any given vehicle requires a certain amount of horspower to make a certain speed. therefore, if the engine is operating at is best, then it doesn't matter whether the fuel is metered by a carb, a computer, a hole in a bucket, or a kid with a squirt gun, mileage is based on engine design, and vehicle requirements.

once again, if your carb is a turd, then fi will be better.....BUT
ever had a map sensor or TPS go bad? or an injector o-ring around a TBI injector start leaking?
that makes crap mileage too.....

tall skinny tires and tune the crap out of whatever you buy

peace, Dave
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Old 02-08-2006, 11:41 AM   #7 (permalink)
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True all that-but isn't it also true that the computerized F.I. is capable of metering that fuel more precisely? Of course there are more variables at work here-the weight of the 4WD units is going to prevent these Jeeps in question from ever being the mileage champs.I think maybe the CJ w/304 with a F.I. conversion may be the winning combination of power and driveability on and off road-any one have any mileage figures on that?For that matter how does a carb w/off-road needle and seat upgrade do off-road?
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Old 02-08-2006, 12:45 PM   #8 (permalink)
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This would be about my remanufactured engine I put in my 88YJ in December. Motorcraft2100 carb and TFI ignition.

When it was installed, the EGR was replaced by a block off plate, and the coolant was not routed through the intake.

The EGR valve was bad, and "wasn't really needed anyways". Since the EGR was gone, the coolant wasn't routed through the intake manifold. Just to the heater core and back.

I have been having a hard time with this engine. Getting good mileage and performance. It is winter and I don't think I've EVER seen a 258 run well in the cold.

I read up on what EGR does. Sounds like I need this working. I read where EGR should only be disabled if you are drag racing. I aint, so I guess I need to hook all this shit back up.

To do this. I will need a new CTO(ported vacuum switch) a new EGR valve, and hook up the coolant lines to route through the intake manifold.

Before I do this I want to know if this will really make a difference.

I thought by not routing coolant through the intake that would make the air cooler and help the engine. Is this not the case on a carburated I6?

If I route the coolant through the intake what is the best way to "burp" the system? My cap has one of those tabs to let off pressure will using that work? Or what is the best way to get air out of the cooling system?

Will installing a tee fitting on the ported vacuum line off the carb to this CTO reduce vacuum to the distributor? Should I look for another source of ported vacuum?

Just looking for any ideas on getting this thing to run better. Or should I leave the engine alone as it is and it will do better once temperatures warm back up?
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Old 02-08-2006, 01:16 PM   #9 (permalink)
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On the I-6 the manifolds don't pick up any heat from the motor so you need some way of heating that manifold or the venturi effect will cause a possible freeze up.The only AMC I-6 I ever had was a 74 232 (cousin of the 258) and they had an exhaust stove under the carb w/heat riser.If there is any way of getting coolant back in there to warm things up I would.EGR does have a beneficial effect on an engine that is designed for it so yeah if you can hook that back up I would.In theory teeing off the vacuum line should not reduce vacuum to the distributor since it should be vacuum tight and just pulling on a diaphram-make sure you have the hoses routed to the correct ports on the switch and the carb.I just got my 75 304 running properly and it has the egr disconnected and I am looking into hooking this stuff back up if I can get the CTO s and what not to do it.I've noticed that it is very slow to warm up and seems to need the manual choke on for a long time or it's not happy-not sure if they have an exhaust manifold heat running under the carb and it's suffering because the heatriser was removed when the headers went in-but it would give the same effect as what you are experiencing.
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Old 02-08-2006, 01:34 PM   #10 (permalink)
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If you have a Jeep, get over the mileage.
If you want a Jeep, get used to the mileage.
If you want the mileage, get a Honda.



On a side note, I have a 258 with a Holley 390, Offy intake, 4.0 L head, GM HEI ignition, and a Cherokee header. I get around 16-17 with a 4 speed, 35's, adn 3.73's.

Also, did you ever take into account the 32" tires and the odometer difference?
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Old 02-08-2006, 03:28 PM   #11 (permalink)
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With my 1985 CJ7 I6 Auto with 31" tires I would get between 11-14. (But if I was planning on keeping that motor I would deff look into upgrading the carb, maybe a webber.)
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Old 02-08-2006, 04:24 PM   #12 (permalink)
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Quote:
Originally Posted by uglyjeepoffroad
If you have a Jeep, get over the mileage.
If you want a Jeep, get used to the mileage.
If you want the mileage, get a Honda.



On a side note, I have a 258 with a Holley 390, Offy intake, 4.0 L head, GM HEI ignition, and a Cherokee header. I get around 16-17 with a 4 speed, 35's, adn 3.73's.

Also, did you ever take into account the 32" tires and the odometer difference?
Do you have coolant heating the intake manifold?
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78 Jeep CJ-5 304 ---setting off car alarms with extreme prejudice.......

out cali style!!!!!!
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Old 02-08-2006, 05:07 PM   #13 (permalink)
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My 304

My cj7 that was STOLEN 3 months or so ago had a rebuilt 304, 3 speed with 2000 miles, not much of a lift and i think 31's was getting 15 mpg mostly city driving. had the mc2100 carb also new. Would of gotten better mpg with a fourth gear. i drove with a light foot, if i tromped it it screamed, had a cam and headers, i miss it

Not sure this has anything to do with the thread though
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Old 02-08-2006, 07:59 PM   #14 (permalink)
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I feel for you though man-I cant wait to get my hotrod 304 on the road ,but I will put some thought into keeping it from getting stolen now that you've made me think about that.
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Old 02-09-2006, 12:35 PM   #15 (permalink)
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Quote:
Originally Posted by NE-RokToy
A properly tuned 258 can get very good gas milage. A good ignition system and properly working carburator are the two main things to address. After that drive like you want to save gas and you should be pleased with your MPG.

I know several people that were very happy with Jacobs ignition parts on their CJ's, noticably improved power and MPG. Two of the guys sould the CJ's for YJ's to get rust free jeeps with EFI and were dissapointed with the loss in power AND fuel milage, though EFI is definatly better in the rocks. The basic setup of all the jeeps in question was almost exactly the same

I already know I'm dumb but can someone please connect the dots for me on how improved ignition improves mileage. I think I understand how a better spark will get more complete combustion and, therefore, more power, but I don't get how that changes the amount of fuel metered into each chamber to be ignited by that better spark.
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Old 02-09-2006, 01:49 PM   #16 (permalink)
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I have a 4.6 stroker, (258 bored and stroked) The best I have got is about 18-19 but thats just cruising. The average is probably 11mpg. Running 35"s with 4.88's and a AX-15 tranny.
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Old 02-09-2006, 04:10 PM   #17 (permalink)
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Quick thought-w/o writing a book on it-if you put on bigger tires-then your speedometer is running slower-the miles are racking up slower-it'll look like you are getting better mileage.The heavier tires will take more power to accelerate-same way they are harder to stop,bad for fuel mileage.I've noticed that people don't really track their mileage that much-it's just kinda like what they think they are getting-and that can be a few mpg off the reality.I've noticed most 4WD vehicles get around 10-14 mpg but few people are willing to admit that.
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Old 02-09-2006, 08:38 PM   #18 (permalink)
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15 mpg in my 258 5 speed cj stock gears and 31's
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Old 02-10-2006, 09:47 PM   #19 (permalink)
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Do you have the egr and coolant hoses to the manifold hooked up?
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Old 02-10-2006, 09:57 PM   #20 (permalink)
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Do you have the egr and coolant hoses to the manifold hooked up?
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Old 02-12-2006, 04:45 PM   #21 (permalink)
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Figure on 14 being good mileage for your rig. Here's a cheap trick that really works. Every time you change your oil put on a brand new air filter element. It'll be good for 2-3 mpg over what I mentioned. Or get a K&N or equivalent. They pay for themselves and the engine even runs better as a result. I put a hotter coil on mine ( a Bosch 50,000 volt.) I could never tell whether it helped gas mileage - but it did make the engine run smoother.
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Old 02-12-2006, 04:56 PM   #22 (permalink)
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i was getting 15-20 mpg running the 258, t-156, 2.73 gears, and 31's. high way was usually around 18
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Old 02-12-2006, 07:27 PM   #23 (permalink)
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Not trying to start any arguments here-just trying to make a point.You being one of the few people who-like myself-will track every gallon of gas that they buy for their new vehicles would understand what I'm talking about.I don't know anybody else who does that-there may be more on this forum,but generally speaking ,most people don't do that.I guess I was speaking about full-size vehicles but if a small pickup will do a couple more miles to the gallon I don't think that's any reason to notify the newspapers. A Ranger is one of the smaller 4WD vehicles and these newer vehicles will certainly get better mileage than the older ones did.It all depends on where you are driving at too-around here it's stop and go and heavy traffic during the week all day long.But let me ask you this-what do you think a full-size 4WD Pickup or SUV get mileage wise?I just took a trip in a 4WD Ram turbo-diesel of about 8 hours-we could get around 13 mpg on the hiway w/ an empty trailer;trailer loaded (w/my Jeep) it dragged mileage down to under 10 for the entire trip average.There was a trip computer on the ceiling from the factory.A Jeep is a smaller 4WD also/with a 4 cyl. engine I would hope it would get better mileage.I was mainly talking about larger vehicles or modified smaller ones I guess.
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Old 02-23-2006, 03:40 PM   #24 (permalink)
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1990 yj AMC 258 with 4.0 head conversion bored .060 over, holley 2bbl #64 jets #6.5 power valve ...1500 miles on the motor 33 inch tires ax-15 5 speed stock dif gears 11 - 12 mpg city, I havent driven it enough on the highway to know about that... Im not sure that I have the carb right yet ...but I cant go leaner than #61 jets or it runs crappy
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Old 02-24-2006, 12:25 AM   #25 (permalink)
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my I6 with a 1bbl carb & a header gets 16mph on the highway with 35's.
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