Jeep Commander Forum banner

1 - 16 of 16 Posts

·
Registered
Joined
·
21 Posts
Discussion Starter #1
Hi

I got my Commander a couple of months ago, and the MPG was consistently horrible (mostly 8.3 MPG and some lucky times I get 10) and I don't know whats the problem, its well maintained, no DTC errors at all, instrument panel doesn't show errors, tires at the right pressure,
its really freaking me out since some people say they get 15-16 which is twice as much as I get, and some 20+ .

is there any known issues causing such drastically low MPG.

P.S my foot is pretty light on the gas pedals and the roads I usually use are mostly level.

Specs: 2007 Rocky mountain edition, 3.7L V6, not many options.
 

·
Registered
Joined
·
143 Posts
That's terrible mileage. I just bought my Commander less then 2 months ago and mine has a 5.7.... but it still gets about 11.3 mpg or so in town.

I'm sure there is either a few usefull threads on that very issue (via the search option) or some knowledgeable guys here will help you out.
 

·
Registered
Joined
·
55 Posts
My gas mileage after a few months was around 11.5-12.5 but alot of stop and starting at lights and slower traffic. I thought the same thing. So since then I put in a K&N air filter only and like you tire pressures are fine. My gas mileage went up to about 14.3 but sometimes dips below that. I would suggest chling you air filter housing and plugs. My air filter house was absurdly dirty and filter was almost totally plugged. Also Ive read that using synthetic oil on changes helps as well. Thats my next move when its due...I will get a K&N oil filter and buy some Mobil1 synthetic 5w-20 oil. Hoping that will help a little more. Changing the plugs this weekend as it has 61k original miles on them even though garage says they can go 100k I'm not waiting till then as buying from the dealer they really didnt touch much on my XK so Im being pro-active.
 

·
Registered
Joined
·
1,585 Posts
Have you hand calculated your MPG?

EVIC (the digital readout) is wrong on many vehicles. If you live in a cold climate and let your vehicle idle to warm up for a long time that effects your MPG. Thats lots of fuel burned and no miles traveled.

Also if you have no changed your plugs yet. Look into that. Every 30k miles ...


I Average 18MPG with a lift and 32" tires. I get 21+ HIGHWAY.


Reset your EVIC MPG readout and see what happens.
 

·
Registered
Joined
·
3,037 Posts
...I got my Commander a couple of months ago, and the MPG was consistently horrible (mostly 8.3 MPG and some lucky times I get 10) and I don't know whats the problem, its well maintained....
How do you know it was well maintained? You've only owned it a couple of months? Unless you've got some pretty convincing service records, I'd be suspicious.

luckyse7ens has several good suggestions. Clogged up Air Filter or Eroded away spark plugs, 10's of thousands of miles overdue to be changed can cause a drastic decrease in mileage. Unless you've got an unimpeachable record of the plugs/filters being changed and are NOT overdue for a change, I'd be checking those items.

And like people suggested, if you're measuring your mileage just on what the EVIC spits out, you're doing it wrong. You need to do the old Fill your tank, note the mileage, fill your tank, note the mileage and numbers of gallons to fill the tank, do the math. Average it over a couple of dozen fill ups to get an accurate mileage.

Finally learn how to reset the average mileage indicator on the EVIC, if the previous owner never did that, your couple of months of driving is NOT going to make one iota of difference on an average that has been running for 8 years.
 

·
Registered
Joined
·
143 Posts
Since I bought my Commander I find myself constantly resetting the EVIC so I can get a real feel report on what kind of fuel I'm consuming when on hills, straight-aways, etc.

Its interesting to watch what its showing but like others have already suggested, record your mileage and do the math. For example on my 07 with a 5.7, has a 21.1 gallon tank. My last tank of 87 octane took me about 268 miles till I decided I really need to get fuel. The message on the instrument panel was showing anywhere between 0, 3 and 6 miles left till empty, so I think I ran it within a half gallon of dry. It came out to average like 12.6 miles to a gallon.

I'm sure that when I change my plugs it will give me an improvement...least I hope to get it to around 14mpg average.
 

·
Registered
Joined
·
1,585 Posts
Have you hand calculated your MPG?

EVIC (the digital readout) is wrong on many vehicles. If you live in a cold climate and let your vehicle idle to warm up for a long time that effects your MPG. Thats lots of fuel burned and no miles traveled.

Also if you have no changed your plugs yet. Look into that. Every 30k miles ...


I Average 18MPG with a lift and 32" tires. I get 21+ HIGHWAY.


Reset your EVIC MPG readout and see what happens.
I Average 18MPG with a lift and 32" tires. I get 21+ HIGHWAY @~65mph, if your cruise speed is 75-80 your getting worse MPG.

You have a roof rack? Anything on the roof kills highway MPG. I removed my cross bars even.

You have the air deflector under the front bumper in place? I have mine installed.

My XK is 2wd, which is less weight and less drag/driveline loss.

I have a hypertech tuner.

Wheel and tire weight probably contribute I have 18x9's with P255/70r18 Bridgestone Dueler RHS's. A narrower wheel and tire combo would be better, but a less aggressive tread (like mine), and lighter sidewall (P is lighter than LT) are ideal for MPG.

Full synthetic oil. Pennzoil Platinum 5w20 atm with mobil one filter for longer OCI's

New NGK plugs.

Driver MOD: My wife's DD. She has a light foot.


WHen I got this vehicle @~90k with stock height, fortezas, and OLD plugs it averaged ~14mpg.

Each of those things likely add .5MPG or less, but all told it makes a difference.
 

·
Registered
Joined
·
143 Posts
The tanks have a about a 4 gal reserve. So when your message says 0 miles or your fuel light comes on you have 4 gal left to run the Jeep. Pretty much we have a 16-17 gal of usable fuel.

I dont want to hijack his thread, but I didnt realize the tank reserve was that much fuel. If that's the case my 268 miles on a tank ..say I only used 19 gallons or so...which jumps me to about 14 mpg or better. That's more where I thought it should be.

Ok..I'm done thread hijacking now. :cheers:
 

·
Moderator
Joined
·
1,993 Posts
I dont want to hijack his thread, but I didnt realize the tank reserve was that much fuel. If that's the case my 268 miles on a tank ..say I only used 19 gallons or so...which jumps me to about 14 mpg or better. That's more where I thought it should be.

Ok..I'm done thread hijacking now. :cheers:
That's not how this works, That's not how any of this works :icon_lol: I couldn't resist. You start with a full tank. Then when you fill it up again you divide that number of gallons with you mileage.
 

·
Registered
Joined
·
143 Posts
Reminded me of that commercial on TV

Ok..so your saying I shouldn't be thinking about how there's still gas inside the tank?

If I understand correctly... If I fill up to max and say I go 168 miles then close to empty my light now comes, showing that the tanks low, didn't you say...
" The tanks have a about a 4 gal reserve. So when your message says 0 miles or your fuel light comes on you have 4 gal left to run the Jeep. Pretty much we have a 16-17 gal of usable fuel."

That would mean that the 168 miles of driving was accomplished on something like 17 gallons of gas, if your explaining that there may be another 4 gallons still in the 21.1 gallon tank. So 168 miles divided by 17 gallons is something like 15.76mpg because we still have 4 gallons left.

My mileage keeps going up...now I'm really feeling good :4:

I'm I confused? ...Sorry....I'm hijacking again :burnout:
 

·
Registered
Joined
·
3,037 Posts
I think its more fuel level sensors that are hard/expensive to calibrate, and the fact the manufacturer errors on the side of showing less gas in the tank then more, to avoid people running out of gas. Where the 1/8th of tank, low fuel warning goes off, and the EVIC estimate to how many miles till empty, I'm sure can vary by several gallons. That is the case with most vehicles, and if the variance is smaller its because they have a smaller tank.

The tank capacity is 20 gallons, that doesn't include the filler tube going to the tank, you can often get another gallon of fuel being held in the filler tube going to the tank.

I watched a jerk throw a huge fit at a gas station because he had a 12 gallon tank, came in running out of gas, and when he filled up, it showed almost 13 gallons. I explained to him the filler neck can hold up to another gallon over the spec, he wouldn't believe it. The attended did the calibration check of the pumps showing they were dead on right in front of him, nope, he kept insisting the pumps were rigged and the gas station was ripping people off.
 

·
Registered
Joined
·
180 Posts
Reminded me of that commercial on TV

Ok..so your saying I shouldn't be thinking about how there's still gas inside the tank?

If I understand correctly... If I fill up to max and say I go 168 miles then close to empty my light now comes, showing that the tanks low, didn't you say...
" The tanks have a about a 4 gal reserve. So when your message says 0 miles or your fuel light comes on you have 4 gal left to run the Jeep. Pretty much we have a 16-17 gal of usable fuel."

That would mean that the 168 miles of driving was accomplished on something like 17 gallons of gas, if your explaining that there may be another 4 gallons still in the 21.1 gallon tank. So 168 miles divided by 17 gallons is something like 15.76mpg because we still have 4 gallons left.

My mileage keeps going up...now I'm really feeling good :4:

I'm I confused? ...Sorry....I'm hijacking again :burnout:
There is no need to use any assumptions in this calculation (e.g. reading E means you used 21.1 gallons of fuel). When you fill your tank next time, take note of your miles, as it sounds like you're doing. Drive until you go to fill up next time, and use the exact figure shown on the gas pump to return your tank to full to divide into the miles you've driven. The only error introduced here is the calibration of the fuel pumps, which should be dang close.

If you were using the fuel tank capacity to divide into your miles driven, your actual MPG will be greater than you initially calculated, but your only accurate calculation will come from eliminating the assumptions. :)

As for the OP, definitely go through with your basic maintenance items. Replace the plugs, air filter, oil, run a bottle of techron through your tank and get the basic maintenance up to date before worrying about where to go next.
 

·
Registered
Joined
·
3,037 Posts
Oh, and I remember one more, check over your exhaust system for leaks and cracks, especially from the engine down to the Catalytic Converters. If there are large leaks or cracks, air will get into the exhaust*, the air will be sensed by the O2 sensor which will make the PCM erroneously conclude the engine is running lean, and thus erroneously richen the A/F mixture a tremendous amount to compensate.

This is a common symptom of cracked exhaust manifolds or bad exhaust leaks upstream or close to the O2 sensor, the mileage takes a nose dive because the EFI reacts to it by richen the A/F ratio a massive amount.

*For those about to say, "The exhaust pressure is higher than the air pressure, no air would leak in, just exhaust out". NOT EXACTLY, the exhaust pulses tremendously in the exhaust manifold, valve overlap in the engine will create a short vacuum surge in the exhaust manifold as well in many conditions, this all combines that air will get sucked in through cracks and bad leaks in the exhaust. Once you get down by the CAT or the muffler, the surges even out enough its pretty much overpressure the whole time.

On the mileage measurement by notes during full ups. You are simply measuring the amounts of miles you've driven since the last time you filled the tank, if the tank was full the last time you noted the miles on the odometer, then the amount of gas you burned can be measured by how many gallons it take to fill up the tank again. i.e. if it took 17 gallons to fill the tank, that meant you've burned 17 gallons of fuel since the last time you filled it up and noted the miles on the odometer, thus it would be the number of miles you drove since last fill up (the current miles minus the miles noted at the last fill up), divided by the number of gallons you put in the tank this fill up.

The pumps are calibrated very accurately, they have to by law and are often checked, gas stations would get in big trouble if they are off. You biggest variable for error in the measurement, is the level the pump shuts off and that is controlled by you as well, as we all vary in how much we squeeze the pump a couple of extra times after the pump shuts itself off. There can be as much as 1/4 to 1/2 gallon difference between just filling it to the bottom of the filler tube to the top of the filling tube with gas spilling over. (And gas foams as you fill it that effects the shut off and spill over). The way you eliminate that variable and get a more accurate mileage reading is to keep doing the mileage measurements the same way and average them, once you've average 5 to 20 of them, you'll have a pretty darn accurate measurement of mileage.
 

·
Registered
Joined
·
143 Posts
Sorry Tarik for what happened to a thread about your 3.7 getting poor mileage. Hopefully someone will read this down the line and benefit from the discussion. And hopefully something we discuss help you out as well.

That being said..thanks for all the good info on capacities and mileage. I read that in 2007 the Commander's tank size was increased to 21.1 gallons, over the 2006 that was 20.8 gallons. I'm not sure if that's totally accurate but that what I had read.


For some reason I kept typing that I was getting 168 miles...I meant to type 268 miles per tank...so I just wanted to clarify that...huge difference.

So, this morning I added 16.31 gallons to my tank. That being said, the gas guage needle was just at the point where it was about to touch the small line... right before the empty line. I'm guessing the light would have been on within the next 10-15 miles or so. The other day when it was fluctuating and was showing 0, 3, and 6 miles left, I had put in around 17 gallons.

So if I do my math, my average on 16.3 gallons of gas come out to getting about 16.4 miles per gallon on the average. So my rough figures are like 15.7 to 16.3 average miles per gallon depending on what I'm doing. I'm going to have to do it over the course of a few tank fulls and see where I'm at.

I now follow that you dont worry about what ever fuel is unused (still in the tank) because it really never enters into the equation when you figure out miles per gallon. As long as your consistently squeezing the maximum amount of gas into the tank every time you fill it up.

And yes..I can easily see an extra 1/2 gallon or so if you max it up all the way up the filler neck.
 

·
Moderator
Joined
·
1,993 Posts
Reminded me of that commercial on TV
That commercial was the first thing that popped in my head when I read you post LOL
So if I do my math, my average on 16.3 gallons of gas come out to getting about 16.4 miles per gallon on the average. So my rough figures are like 15.7 to 16.3 average miles per gallon depending on what I'm doing. I'm going to have to do it over the course of a few tank fulls and see where I'm at.
Now your getting it :clap: 268 miles traveled divided by 16.3 gallons pumped in equals 16.4 MPG
 
1 - 16 of 16 Posts
Top