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Discussion Starter #1
The point of this thread is not to argue about what octane to run or whats best, it's simply a case study that others might find helpful.

Some details on the Jeep:
2007 Limited Hemi QD2
245/70/17 General Grabber AT/2s @35 PSI
~30k with fresh plug change (used OEM coppers w/correct gapping)
Superchips Tuner

I have a consistent 25 mile round trip to work that has 6 traffic lights. Street speed limits vary from 35 to 50.

With the 91 Octane performance tune and running top tier 91 octane gas i was getting 12.5 MPG.... maaaybe would see 12.6 if I never got stuck at a red light. This has been consistent for the past 3 months (when the Jeep became my DD).

I decided to re-tune my Jeep this week to 87 Octane.

First thing, i knew my speedometer was slightly off with the 91 tune. I put in the calculated tire size of 30.5 and i originally tuned it and my GPS always had me going slower than my speedometer said. When I re-tuned my truck with the 87 octane, i measured actual tire height and got 30.0 (damn near exact). My speedometer is now correct.

Adjusting for the slight difference in tire: (30/30.5)*12.5 = 12.3

So for the comparison, my 91 octane tune was actually yielding 12.3 MPG since i was not actually traveling as far as my odometer thought.

After 3 days of commuting with the new 87 Octane tune and running 87 from the same gas station, I'm getting 13.1 MPG.

One note: when i filled up my tank took 13.5 gallons of 87, the remainder in the tank was 91. So i'm not running pure 87 yet. I'm interested to see how my gas mileage changes with subsequent refills.

I'm off the rest of the week but I plan on continuing to run 87 to see if anything changes. Once I have been running 87 long enough to really get a solid feel for my actual MPG, i'm going to start running 89 to see if there is any effect.

In any event... i can't remember the last time I saw the number 13 in my MPG screen. I have however seen 11 on occasion; sometimes I just catch all the red lights for an entire week.. sucks when that happens.
 

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Thanks for the research effort. It will be interesting to see more results.
 

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Discussion Starter #3
Looks like I was wasting a lot of money with the 91 Octane.

My gas mileage stayed right around the 13mpg mark for my daily commute with 87 in the tank. Worst I saw was 12.7 (when stuck in a lot of traffic) and there were days when it was hovering around 13.2.

I switched over to 89 Octane yesterday... so we'll see if this changes it at all. I'm hoping for a solid 13-13.5, that would be nice :)

Summary:
91 Octane w/91 Performance tune = 12.3 MPG
87 Octane w/87 Octane Performance Tune = 13 MPG
89 Octane w/87 Octane Performance Tune = TBD

** This is with my normal daily commute. Highway gas mileage seemed to have improved also (around 16) but I don't do enough highway traveling to know for sure if there was a change.
 

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Good write up, Matt!! We started running 87 in my wife's 08 Hemi Charger(which got traded for my jeep), and other than some barely noticeable power loss, it was fine and mileage appeared to be same or slightly better.. I've tried it in the Jeep, while it's not "tuned" for 87, I'm not seeing any ill effects..Keep us posted, as I'm considering buying a tuner and wondering if it's worth it for the limited amount of miles it gets driven
 

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Discussion Starter #5
I bought the tuner primarily to adjust the speedometer for the increased tire size and for a few of the standalone features that come in handy for offroading or maintenance (like being able to increase the idle RPMs for running a winch or to heat up the engine quicker after a coolant change).

I've heard of people getting better gas mileage with these performance tunes over stock... and when i was seeing 11 on occasion and never getting above 12.5 i decided to start this little experiment. I'm a bit happier with 13 :eek:rangehat:
 

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Obviously if you're talking about cost, we need to factor in the cost of 87 vs. 91. And for those looking solely for mpg increase, you need to factor in the cost of the parts that save you fuel (programmer, intake, exhaust). $1000 is a lot of extra gas.

I wish the programmer had a "Super Econ" mode that kept the engine in 4 cylinder mode longer.
 

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Discussion Starter #7
I believe there are "economy" tunes that you can get for these.

But to be clear, i did not buy the programmer for mpg.

All i'm doing here is recording what my actual MPG figures are with the different tunes and octane levels.

I've seen multiple threads where people claimed that with the 91+ tunes they were getting enough of a MPG increase to offset the price of the fuel. That obviously isn't the case for my vehicle/commute.

There's also been a lot of questions about whether to run 87 or 89 in the 5.7L. Since there's not a clear answer to that, im doing my own test to see if there's an effect on MPG between the two. (Edit: if the 87 and 89 yield the same results, i'll probably keep running 89 per the owners manual since it may actually make a difference under hard acceleration or towing; neither of which I do on my daily commute. Well... i do hit the gas once in a while... but not every time the light turns green)
 

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I just got mine updated, so I'll be checking out the new features today.

The cost vs. mpg thing was just a word to the wise.
 

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Matt, good numbers man. Our 2010 Hemi is getting considerably better mileage than that though. With the wife driving it mostly, it's getting 15-15.5 in and around town. On the highway, if the wind is mild and without too many hills, it gets 19-19.2. I hand calculate mine at every fillup, and it is almost always within a couple tenths to the EVIC. It's still on the factory 29" tires, and has the 2.5" RC lift and a Predator tune on the 87 tune and a drop in K&N. After 4700 miles, the EVIC is showing and average of 16, which I don't complain a bit about for a heavy 4wd suv.
 

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Matt, good numbers man. Our 2010 Hemi is getting considerably better mileage than that though.
The 2009 HEMIs had some updates including staying in 4 cyl mode longer.

The 2009 Jeep Commander will arrive in showrooms this September with some welcomed new features. The current 5.7L Hemi engine will be replaced by the all-new 2nd generation 5.7L Hemi. The new Hemi offers variable cam timing; a higher compression ratio; higher-flowing heads, intake, and exhaust; and an active intake manifold that switches from long runners to short runners, to optimize the engine for either better high-end horsepower or better low-end torque. Horsepower output is expected to be around 370, with torque numbers near 400.
 

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I run 89 all the time in my 06. I drive 50 mi one way to work. If I drive 65/70 I get 13/14mpg. If I drive 55/60 I get 16/17mpg. It is all highway at 3:45am So no stop and go just me and a few other cars on the road. sometimes I dont even hit my brakes till I get to my exit. I would like to get a tuner. What tuner do you have?
 

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My 2006 4.7L XK is getting 13/16 after putting on SpiderTrax 1.5" spacers, an RC 2" lift, and Scorpion 265-70-17's..Before the mods I was getting more like 16/19. After reading a bunch of posts, guess I should just be quiet and enjoy my customized Commander :) Which I do very much...
 

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Discussion Starter #13 (Edited)
Final update: Looks like 89 Octane is what my vehicle wants to drink. Haven't seen the number 12 since I put it in my tank... and have been getting closer to 13.5 during my commute. Lowest reading I got was 13.0 (I was watching carefully to see if it'd hit 12.9 but never did).

Summary:
91 Octane w/91 Performance tune = 12.3 MPG
87 Octane w/87 Octane Performance Tune = 13 MPG
89 Octane w/87 Octane Performance Tune = 13.3 MPG


Looks like I was wasting a lot of money with the 91 Octane.

My gas mileage stayed right around the 13mpg mark for my daily commute with 87 in the tank. Worst I saw was 12.7 (when stuck in a lot of traffic) and there were days when it was hovering around 13.2.

I switched over to 89 Octane yesterday... so we'll see if this changes it at all. I'm hoping for a solid 13-13.5, that would be nice :)

Summary:
91 Octane w/91 Performance tune = 12.3 MPG
87 Octane w/87 Octane Performance Tune = 13 MPG
89 Octane w/87 Octane Performance Tune = TBD

** This is with my normal daily commute. Highway gas mileage seemed to have improved also (around 16) but I don't do enough highway traveling to know for sure if there was a change.
To reiterate my point here:
This thread is not about "what gas mileage does everyone get". It's a case study of what my particular vehicle with my mods, my commute and my driving style get with variations in Octane and the SC tuner.

You really can't compare gas mileage between different people and vehicles... we all drive differently, drive different places, have different tires, tire pressures and mods.

So what's the point? Well... say you're running the 91 Octane tune on your 5.7L HEMI and getting 14.5 MPG....you may think "whatever, im getting better gas mileage already". But based on my results, you may be able to get 15.5 MPG if you put in the 87 Octane tune and run 89 Octane fuel. So it's worth a shot and I have some data to back it up (an increase of 1 MPG and a decrease of $.30/gallon can add up quick):eek:rangehat:
 

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Matt, good numbers man. Our 2010 Hemi is getting considerably better mileage than that though. With the wife driving it mostly, it's getting 15-15.5 in and around town.
The 2009 and up HEMIs had some updates for more power and fuel mileage.
[XKJeeps.com, Sept. 2007, updated Feb. 2008] The 2009 Jeep Commander will arrive in showrooms this September with some welcomed new features. The current 5.7L Hemi engine will be replaced by the all-new 2nd generation 5.7L Hemi. The new Hemi offers variable cam timing; a higher compression ratio; higher-flowing heads, intake, and exhaust; and an active intake manifold that switches from long runners to short runners, to optimize the engine for either better high-end horsepower or better low-end torque. Horsepower output is expected to be around 370, with torque numbers near 400. In addition, the MDS system has been updated so the engine will spend more time in four-cylinder mode, saving fuel. Numerous other hardware upgrades were implemented to build on the quality, reliability and durability reputation of the HEMI engine, including crankshaft structural upgrades, a dual-mass crankshaft damper, floating pin piston design, valve spring design, and oil pump capacity increase. As a result of the new updates, gas mileage is estimated to be increased by nearly 5%
 

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I "think", but do NOT know,

Lower Octane fuels vaporize more readily and that result in more complete burning in the cylinder, i.e. gets you better mileage.

Higher Octane fuels vaporize less readily and that results in harder cold starting especially in very cold weather, Chrysler has a TSB out on this, that most of the cold stalling problems from customers come from them using higher than recommended octane fuel in cold weather. It also results in the fuel being less likely to auto-ignite on its own too early in high performance motors.

You are NOT the first one to notice better mileage with lower octane fuel, I've seen people post the same on other forums.

If you're using the recommended octane, or the lowest in the range of recommended octane, you should be perfectly safe, Some things to keep in mind, especially if you experiment with lower than recommended octane:

If the octane is too low for conditions, it will cause Knock or Detonation, the PCM will recognize it and retard the ignition timing to stop it and keep the ignition timing retarded for a while. Retarding Ignition timing can result in much lower mileage. So track your mileage, if lower octane, especially lower than recommended is resulting in worse mileage, this is likely the cause.

Knock and Detonation can cause serous engine damage, if your using lower than recommended Octane, know how to spot them and reduce throttle if you encounter it. Yes, the engine has a knock sensor and the PCM will combat the knock if it senses it, but if you're using lower than recommended octane it is possible that even the PCM retarding the timing can NOT stop it and you could damage the engine if you keep the same throttle setting and load with the engine making popping corn noise, knocking and pinging.

The potential for knock or detonation varies with all sorts of factors, the Octane recommendation is really for the worst case scenario and the PCM knock sensor is there to protect you in the unanticipated conditions. Thus people use lower than recommended octane in favorable conditions and never suffer a problem, DO NOT ASSUME that means the lower octane will work just as well in all conditions.

The biggest, but NOT all of, the factors is Air Density/Pressure and Air Temperature. Hotter and Higher Pressure the Air, the more likely you are to get detonation.

So folks drivining in the mountains at winter in an empty vehicle use lower than recommended octane all the time and its perfectly safe for their motors.

If you're going to tow a heavy load in death valley in the Summer, I would use higher than recommended octane.

You been using lower than recommended octane all winter, don't assume that it will work just as well in the hottest days of summer.
 

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Discussion Starter #16 (Edited)
The intent of this was Not to experiment with lower than recommended octane levels. (FYI the owners manual for the 5.7L says 87 can be used if pinging doesn't occur, but 89 is recommended for best efficiency and that they DONT recommend running 91. Based on my tests, even with the engine "tuned" for 91 via superchips, the owners manual is spot on correct)

Higher octane fuels actually have less energy per unit volume than lower octanes.... most people don't realize this. Higher octane fuels RESIST predetonation, thus their effectiveness comes in the form of allowing detonation at a higher effective compression ratio which in turn increases the efficiency of the Otto Cycle. This increase in efficiency quickly overwhelmes any decrease in energy per unit volume.

The key is, higher octane fuels are useless in engines not designed or properly tuned for them. This varies engine by engine.

My goal was to find which combination of programming and fuel octane performs best for the 5.7L (or 'my' 5.7L).

Too many "enthusiasts" are somewhat blind to their mods... everyone seems to think you need to run the highest octane, which isn't true in every case. Hell I can probably start selling a "low friction" film for people to put on the front of their vehicles, claim an increase of 10-15 HP and 10% better fuel efficiency and I bet people would buy it and swear it works.

Thus, my point here was to post up some actual "tested" results using as many controlled variables as possible.
 

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Hell I can probably start selling a "low friction" film for people to put on the front of their vehicles, claim an increase of 10-15 HP and 10% better fuel efficiency and I bet people would buy it and swear it works.

Thus, my point here was to post up some actual "tested" results using as many controlled variables as possible.
It is tale marketers to sell us goods more expensive, these really + can not be verified, we must take into account many real conditions including the strength and direction of the wind .................... .
 

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With an AFE cold air, magnaflow exhaust, and the diablo progamer I was getting 19-22 mpg before I lifted my XK, now that its lifted 4" and has 33"x9"s its down to 13-15 mpg with 91 octane. If you are concerned about mieage, don't lift it!
 

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If anyone would want to send me their original backup and current tune I can modify it to get get slightly better fuel economy with the diablosport device, I am pretty sure I can edit MDS calibrations. As soon as the old lady gets over my first round of mods I will be using my own diablosport device to do some testing on my own commander, even though I will be disabling MDS all together (can't stand it).. I'm still waiting for some parts to come in. Just anxious to see what I can get out of these things..As for running different levels of octane in your commander, this can be simply done by changing spark timing according to your fuel.. I have every intention of tuning my commander on 87 octane only unless it gives me a really hard time..
 
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