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Discussion Starter · #1 ·
I was wondering if anyone knows what RPM gives the most efficient fuel operation.

I used to get 20 mpg on an older cherokee with 31's, with manual tranny, and 3:73 gears. I attribute this mostly to the gear setup, and also to the rpms at highway speeds were at around 2,000 rpms.

I get around 10 mpg on my 03 Rubicon with 36's manual tranny, and 5:13 gears. I am fully aware that it is a brick on larger tires, so we can leave that out of the conversations, however what I am looking at is trying to get the best gas mileage with my current setup by modifying the speed at which I drive, not the vehicle.

I had been driving it at 60 mph at around 2,300 rpm thinking that it would get better gas mileage at a lower RPM, but today I did a road trip down the interstate and drove it 70 mph, at about 3,000 rpm and it almost seemed like it was getting better gas mileage. Does this sound correct? Does anyone know what rpm band this engine is most efficient at?
 

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I can't comment on the ROM except to say the lower the better. I will say that Skinny Pedal MCAI will give a slight increase in MPG. Good luck!
 

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It depends on a lot of variables BUT at 2500 or under, you're lugging your engine and can expect worse mileage. I've done extensive testing on my rig over the last 5 years and can average over 17 when I have it tuned right and running right-that's with a 6 speed, 4.88's and 33's. I run at or over 2800 all the time, and mostly at or over 3000, with intermittent runs above 3500 rpms. My rig see's it's optimum efficiency, optimum performance and best mileage there. On a recent trip to the Rubicon, I pulled over 18 mpg on a couple tanks and a tad over 19 on a couple tanks. This was over 2000 miles of highway driving. Like I said-LOTS of variables though and you need to keep things in top shape and pay attention to what you're doing and how you're driving to maintain good mileage.

That said, a few things to help are to maintain proper tire pressure-air up a bit more for long highway trips. New/Clean air filter, fresh oil change, good plugs-I change mine every 25K, and with that, pull the TB and THOROUGHLY clean it and the IAC. I've been running a modified air box from a mustang cobra for a while now which pulls from the cowl, and had been running the exhaust without the 3rd cat and through a higher flow muffler. Due to a bad cat, I had to replace the setup and went with an entirely new cat replacement from Eastern Catalytic. It came with the 3rd cat so I thought I'd put it back in to see how it performed. I will be removing it-the rig did loose a bit of power and about 1-2mpg with the three new cats in. Removal of the 3rd did make a difference prior so I'll take this one back out here shortly.

You should be properly maintaining your rig for best performance, but as you're not interested in doing anything other than drive different, on speed, I do best in the 60-65 range and at or right around 3000 rpms. Depending on where I'm at, that means staying in 5th and not running 6th. In certain areas I can maintain higher speeds of 70 in 6th as long as I keep the rpms up and still realize good mileage.

Again, I'm running the 6 speed with 4.88s and 33's. 5.13's with 36's should be good on the rpm range and would be what I would run if I was that big. I don't expect you'll see quite as good as what I get but you should definitely be doing better than 10.

Best of Luck,

Mike
 

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Discussion Starter · #4 ·
Wow! Great info, Thanks for doing the research! I am running K&N intake and Magnaflow exhaust system, so it should breath fair.

I was thinking the same about the lower RPM's the better while asking, but your research shows differently which is why I asked. Does anyone else have further research/info/opinions on this subject?

Thanks for your time and info!
 

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Take it for what it's worth. But stay away from the K&N setups. When they were all that was on the market, they were good. But now the competition has stepped up and surpassed K&N. Their filters have tiny holes in them. Find one and hold it up to the light, any of them! They all will be the same. Also oil is used in their filters, on some things this is not a problem. On vehicles though this can be a problem. The oil can collect inside the intake tube and on engine sensors thus gumming them up.

Any of the dry filters are better. Personally I run an AEM dryflow CAI with and Outerwears prefilter on the AEM cone filter. Lots better than the K&N I used to have.

The best one for our jeeps though is the MCAI on Skinney's website.
 

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the cherokee you owned was a pretty light vehicle compared to your rubicon. I also owned an XJ before my rubi and it was amazing on gas for a 4 wheel drive vehicle. I too have found with the wrangler higher RPMs seems to get better mpg. every vehicle I have driven if I short shift I get better mileage including the XJ. the wrangler is a whole new animal to me. that kinda blows my mind since the XJ and the LJ have the same engine.
throw some 31's and 373's on the rubi and see what ya get. haha. actually my GF's TJ has 373's and 30's with the 6spd and it gets awesome mpg on the highway compared to my LJ.
 

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I started to start a new topic out of this thread, but decided that this is with in the realm of the OP's topic since he did hit on it some.

Mike & MPETE..... and any other manual transmission people out there...

Your ideas up above are very interesting to me! It boggles my mind of sorts you might say. I have always been under the impression that you keep your tires aired up, your foot out of the floor board with the skiny pedal, and you shift soon in the powerband. "Soon in the powerband" to me is shifting around 2000-2500RPM. Between the two of you, your experiences TOTALLY go against that idea and well ingrained habit of driving I just described (at least the shift points does). I am going to have to try this, I really want to know more about the experiences the both of you have had and where you shift between gears in your rubi. Cause I got to tell you... I know neither one of you are telling a tall tale, but the just sounds soo odd to me. Course apparently it surprises the both of you as well.


Now for reference.... I get anywhere from 11-12 MPG in the city, driving short trips around a small town. I run 89 octane by choice, and I get it from the same pump at the same station most every time. Course manual 5 speed transmission, 5.13 gears, and 35" nitto MGs that are heavy esp. with beadlocks!!
 

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At the risk of getting flamed, I'll give you this as we're all amongst friends here. :D I totally agree with you. It goes against everything I've done in my other rigs as well. My old Honda commuter liked it under 2500 and I could pull over 40mpg in that car. Both of my old '80 4x4 K10 Chev short box liked it around 2200 rpms and my current '78 4x4 K20 long box with 350/400/203 and 35's like it around 2200 as well.

My '07 Subaru 4 cyl boxer is finicky and does best around 2800-3000 but spins around 3500 in 5th at 70mph. No matter what I do to this car, the BEST I can ever get out of it is 28mpg, and 26mpg is average. If I don't clean the TB on that car and keep it tuned like I do my Jeep, it will only see around 22mpg.

I have TWO main daily drivers-my '07 Subaru that I used to commute to work in, that since my baby girl was born my wife now drives all the time, and my '05 LJ Rubi. I commute about 50 minutes to work from the new place and with what I do with my Jeep, mileage IS important to me. Being a poor college student years ago driving a built 4x4 Chev, I did what I could to get the most out of my $20 gas budget-which didn't go far in that truck! :laugh: Because of that, I don't put up with the crap about "it's a Jeep, get used to it". Yeah, it is a Jeep, yeah, it is a Subaru, yeah, it was a full size Chev truck, yeah it was a Honda, but who gives a rats ass-you CAN make them perform to the best of what they'll do with a few simple tricks and regular maintenance. When you're broke, you find ways to stretch the dollar and the difference between 10 mpg in a Jeep and 17 average is HUGE. I'll gladly take a few extra steps to get it up there.

That said, when I was on stock gears and 33" rubber, my rig HATED life. I HATED life. Besides losing 6th all together pretty much everywhere and using 1st as a dedicated starter, and horrible off-road performance, my gas mileage tanked to about 11mpg if memory serves. I couldn't tow my trailer either so life was awful for a while-piss poor performance, piss poor economy, piss poor off-road use. The Jeep was crippled. A simple gear swap to 4.88's brought things way back to being good again in all aspects, and YES, I did gain mileage back. I forget exactly how much, but it was around 3mpg or better.

Now, on your other thoughts, you are correct on a couple things-keep tires aired up and your foot out of it for the most part. That's the funny thing about factory gears that everybody thinks is so great for mileage-it took way more fuel to get up and moving from a stop light with factory gears and 33's than it ever will with deeper gears. You loose a HUGE amount of fuel with your foot in it at a stop light and getting up to cruising speeds than you ever will maintaining cruising speeds with the factory gearing. Factory gearing with taller rubber also draws more fuel to maintain those speeds as the engine is lugging a bit to maintain. At higher rpms up in the "power band" if you will, the engine is operating at peak efficiency making it's most power. Horsepower/torque and mileage go hand in hand as do intake,exhaust,vacuum, air/fuel, etc., etc., etc. You can't rob peter to pay paul and come out even in the end-something has to give somewhere along the way. Faster speeds draws more fuel to get the power it takes to maintain said speed in any given equation up until the engine is running at peak efficiency. At that point with everything in tune you'll realize the most power, best performance and most mileage. Getting there is the trick.

I know every Jeep is different and this does go against everything you'd be lead to believe in other vehicles, but keep in mind this is a Jeep with a 4.0L engine, not a big V8. They say there's no replacement for displacement as which with comes in a larger engine. We have the i-6 and it WON'T perform like a big V8 no matter what you do to it. My rig is heavy in comparison to the old Honda, and heavy still when compared to the Chev trucks that DO have a v8. We won't even bother to comapare it with my '07 F350 superduty flatbed diesel... I know where my rig runs best and that's where I keep it for highway use. Off-road is totally out of the equation and should never be considered or thought of. I don't, and won't ever calculate mileage or expect to get good mileage when wheeling. You shouldn't either-I'm talking strictly highway miles here for a DD rig. That said, 3000 rpms is where MINE wants to be. I'm not an engine builder nor do I understand exactly what is happening in the engine or why, I just know it works.

So with your "power band" and short shifting-I NEVER short shift. My rig spools up quick with the 4.88's, higher flow exhaust and mustang airbox. I run it up to 3000 MINIMUM to shift and often over that upwards of 3500 or so so that when I make the shift to the next gear, it's already IN the power band it needs to be and not lugging to get there. Not sure how to explain it other than do a short shift from 1st to 2nd at 2000 and note how it pulls in 2nd. It will be more noticeable in the higher range so try it again between 3rd and 4th shifting at 2000rpms at a rolling speed of say 20mph. Next time shift it IN it's power range of 3000 or better and note the difference in performance when you drop into 3rd or 4th in the higher rpm range-it will get up and move much quicker than it ever would at 2000. Make sense? Try passing a semi on the highway up hill in 6th gear at 60mph and see how long it takes you to pass said truck. Do the same dropping to 5th, or even 4th if need be, rev it up at or over 3500 before shifting and see how much quicker you can pass said truck. The engine is in it's optimum efficiency range for lack of better terms where performance is best, and consequently mileage will be best too. Now, don't get me wrong, you won't get the best mileage passing semi's uphill at highway speeds, but point being-if you drive your rig on the street lugging all the time, you're essentially doing the same thing... Another example if you have a new style Ford/Chev/Dodge truck to tow with-tow a trailer in "TOW/HAUL" mode and note where the auto transmission shifts and at what range. Same thing in the Jeep.

Please keep in mind this is MY observations driving MY rig. Nobody knows my rig better than I do and I don't expect anybody to drive my Jeep and get the same performance I do with it. Take my wife for instance-she is very in tune with the Jeep and can point things out to me that are wrong that I don't notice. She's often right. BUT the difference between my driving and hers is the equivalent to nearly a 5mpg difference. With her driving-no matter what shape my rig is in, she won't ever get better than about 12mpg, maybe 13 at best. All in driving style. She shifts early and keeps the rpms low no matter what I tell her to do. I can't teach anybody how to drive their rig for best performance. I do know what works best for ME in MY rig with all the testing and playing I've done over the years and some of that is listed above. Please keep in mind this is not the end-all be-all answer to getting the most out of your rig-this is just what works for me. I've played with it a lot over the years and yes, it does go against everything you've been lead to believe... As to something that will blow your mind-on our recent trip to the Rubicon Trail, I had a couple tanks over 18mpg and two tanks over 19mpg. This was over a 2000 mile trip and MANY tanks of fuel. On that same trip, I had a horrible head wind with driving rain and had one tank at around or just under 15mpg. Like I said, LOTS of variables... Your results may vary. :D

Enough rambling, I gotta go hack up the Jeep to build some new parts...

edit-I should also note I run "regular" fuel which I think is 87 octane here. The above is for highway driving-city for me is always lower too due to stop/go driving.

Best of Luck,

Mike
 

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LOL! :D Yes Mike, we are all amongst friends here. Your not going to get :eek:nfire: by me. What you said makes sense to me. It really does. And I am going to just have to try it and experiment some on my own. Like you said, vehicles vary and differ greatly. But it makes me wonder if there isn't a 1-2 MPG still there I am missing. Wouldn't that be a kicker to discover? All this time I thought I was "driving right" and "saving gas" only to find out if I let her wind out a little more it would be even better. I never claimed that my 11-12 MPG was great, but I honestly thought it wasn't bad considering, and I still don't really think it is. But like you said I will just have to try and remember to try your driving style or similar, put my normal way of doing things on hold, and see what numbers I can crank out either good or bad. All in good Jeep'n fun R&D! :laugh: Either way, I am not going to try this expecting a magical cure or over nite change and all of a sudden get the same numbers as you. Too many variables to place those kind of expectations on a mechanical object. Besides, once again, you run into that whole "my jeep is diffferent than yours" ordeal. Weight, gearing, tires, even their pressure and tread pattern, etc, etc.... all play a difference.

I live in a some what "hilly" terrain, so your style of driving makes all the more sense to me, and I understand your examples. It is just one of those things that when you read it, and consider what you have heard and done for soo many years it just sounds so obscure and strange. Not the first time that has happened to me! :laugh: I don't have a full tank right now, but I will have enough to compare and then I will do a full tank. I am anxious to see the results of my own tests. One thing is for sure, regardless if I get a better gas milage reading or not, I'll enjoy every darn bit of it cause I love driving the darn thing.
 

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well to answer your question I generally shift between 2500-3000 NOW. I used to shift around 2000-2500 and it seems like I have gained about 1 mpg by doing so. nothing else about my driving habits has changed. my jeep is totally stock drivetrain wise 05 LJ rubi 6spd sittin on a 2" BB, 1.25" BL. I have 33" MT's and 15x8 procomp steelies.
I find it funny that my jeep is setup EXACTLY like Mike's was and he said his was CRIPPLED.
 

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Discussion Starter · #11 ·
Thank you guys for your help, Keep the info coming, I am axious to see what you find SweetPee on yours as well!

Unfortunately, I don't get much of a chance to do highway driving until I do a long road trip here or there to go to Colorado, most of mine are just short drives, however, on occasion I drive 100 miles each direction, and wish to try to get the best i can with what I have, only changing my habits.
 

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I have been contemplating<< big word.. this for a while now. I don't have any hard numbers yet, but My 06 Tj auto with 4.88 & 35's seems to get better around town with the OD off and not turning it on till I'm up to around 65 getting on the highway. I also just removed the mechanical fan and replaced it with a hi flow electric. I'll be interested to see the MPG difference, there is definately a power increase both low end and high end without the mech fan according to the ol' seat dino.
 

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Discussion Starter · #13 ·
I generally don't drive much faster than 60, even on the interstate, I got the same fuel mileage pulling a 6x10 trailer full of camping gear, as I did running without a trailer,(9-11) but Monday, I was running late for my father's heart surgery, so I took it up to 70 and watched my gas guage, it seemed to go down slower(comparing miles to the guage). (I get a really bad vibration between 62-68 so I avoid that at all cost, but it smooths out at 70 "WOO HOO!!".) I am only sorry I didn't find out sooner, I was afraid the vibration may get worse if I kept increasing in speed. I didn't want parts to start falling off.
 

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The peak volumetric efficiency of an engine occurs at the point of peak torque. In the case of a stock later 4.0 that's 3200 RPM, which is rather high for a "truck" 6 cylinder. RPM is not the enemy of fuel economy with our Jeeps, parasitic (aerodynamic) drag and rolling resistance (big noisy tires) have a far bigger effect.

This is why gearing is probably the best bang-for-the-buck if you are looking to increase your performance.
 

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Discussion Starter · #15 ·
well, you are probably right about that, however, again I stress, I don't want to change my rig, just my driving habits, if it will help get me a few more miles per gallons out of it.

I bought the Rubicon for an off-road vehicle, primarily, but still a family vehicle if needed. The performance I enjoy is the offroad performance, but I since I drive it to and from where I wheel, I want to save money where I can, by speeding up, or slowing down the vehicle, or even changing gears differently if that helps.

After my current tires are worn out, I will likely purchase some more mild tires for it, right now, I am running 36" Iroks, and not only are they really loud, and vibrate like no other, they do have huge drag. I bought the TJ with them on it, I did not pick them out, but sure like to offroad with them.

If anyone is considering them, they are great.. offroad, but I lost 1/4 inch of tread off of them driving to Colorado and back. I knew this was a possiblity, so I took three spares with me, however 1/4 is actually less than I thought I might lose from them considering they are primarily an offroad tire, and how soft the rubber compound is.
 

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remember something..ROTATING MASS is more parasitic than sprung weight. Meaning, the bigger/heavier the tires/wheels are, the more load it actually puts on the engine...than say carrying a fat chick in the back seat :D
 

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This has been gone over before over the years here and other places. My jeep got 18 to 19 MPG with the A/C off and the hard top on, it would drop to 15 with the soft top and down to 10 with no top no doors back to about 17 with doors, tonue covers and sun top, with the stock 3:73 gears and 33s. % inches of lift and I tend to run 65 to 75 mph most of the time. If I had to run 55 to 65 for a trip MPG would go up a bit like 1 to 2 more. With my 5.3 I’m getting about 22 to 24 now at the same speeds and it does not matter what top or if the A/C is no. It would seem that the I6 was just working too hard all the time. Of the Guys I talk to about their Jeeps, and I talk to allot. Most are getting some place around 13 to 15 MPG does not matter what they have done, tires or gearing, hard or soft top. Or even their ages or there driving habits. The one thing that comes out is the I6 motor is a very old motor, Add to it that the 97 to 02 has a fuel mangement system that was old then and did not work well. The 03 to 04 have a 2nd system that was designed around a V6, as was the 05 to 06 system. Is no wonder not much helps. Cold air is the best thing the motor can have if you ever look at the air inlet temp, a scaner to the OBD II port will show you this, you will see temps of 100 to 160 deg. I dont know what the coldair from MAC does but I would bet its a lot lower. Even my jeep air inlet temp is too high, 150 on a summer day. And it get its aire the same place the COLD AIR kits do behind the right head lamp. SO MAC intake should give every one here the best bang for the buck to get milage up. I dont remember but it seems that droping the intake air from 150 to 80 or 90 should get an increse of 25% or more in power. Opening the hood on the dyno testing on my jeep changed the number up by that amount. the only chnage was cold air to the motor.
 
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