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Why is my gas mileage so bad?

29K views 20 replies 10 participants last post by  Lymzilla 
#1 ·
Ok I bought a 2008 RT Charger, and while I am fully aware that I did not buy a Prius, it seems like my MPG's are uncharacteristically low based on what I have been seeing other people getting.
Now I purchased the Charger used and the only physically noticeable modifications when I got it was a K&N Cold air intake and a set of Pypes dual exhaust, but nothing I read suggested these alone would greatly rob my gas mileage.
I am running 91 Plus in it and I checked the on-board fuel economy reader against actual calculations and it is accurate. I spent a week driving (mostly city) but as responsibly as possible, never taking the tac over 1,800 rpm's and not getting there quickly either, at the end of the week I was at 10.4 MPG's and I have pinned cruise at 60mph on the highway for 15 miles and was getting 16 MPG's.
I'm just wondering if I am stuck with this, or if anybody can think of something the previous owner may have done to rob my gas mileage, and if so can it be reversed?
Thanks for any help!
 
#2 ·
Mileage?

First thing I'd do is check the plugs and change, if necessary, to the champion coppers.

They're supposed to be changed every 30k miles.

Also, no reason to run 91, unless you have a tune installed.

Is MDS enabled and working?
 
#3 ·
Only thing I can think of is that the R/T's are supposed to run 89 octane, unless you have your car tuned for a different octane.
 
#4 ·
I have a little bit of the same thing going on. I lock in my cruise control at 60 mph, and reset the MPG monitor, on even to slightly down hill grade. It showed me getting 18 mpg, shouldn't that be 23-25????

Also, my car sounds like it is reving hard while cruising around 77mph, and is around 2500-3000 rpms, are these cruising rpm's on par with others here???
 
#6 · (Edited)
I have a little bit of the same thing going on. I lock in my cruise control at 60 mph, and reset the MPG monitor, on even to slightly down hill grade. It showed me getting 18 mpg, shouldn't that be 23-25????


Also, my car sounds like it is reving hard while cruising around 77mph, and is around 2500-3000 rpms, are these cruising rpm's on par with others here???
No. The best I have gotten was 24mpg and that was cruise set to 75mph for a 1200 mile round trip. I haven't gotten that mpg again since. My average is 17-20 depending on how I am driving. Plus the AWD R/T is a pig even to the already piggy R/T lol.

The rpms are in the right zone. Your FM's are going to exaggerate the sound at cruising speed big time.

I'm assuming these were serious questions and not being silly ;)
 
#5 · (Edited)
Okay a few quick questions to help get some better answers:

Do you have a Predator? If not, you should be running mid-grade gas which is 87-89. Using 91 is high without a tune.

Are there any codes (engine light on)?

How long have you had the car?

How many miles on the vehicle (round number, doesn't have to be exact)?

Do you have service records and/or Carfax or other vehicle history report?

My worst gas mileage is 17mpg and that is when I'm doing really dumb things along with stop/go city driving. 10mpg tells me something is really wrong. Answer those questions and whatever else other people ask and perhaps we can give you an idea of the issue, but as of right now it looks like you need to take your car in to be checked out.
 
#8 ·
No Predator so I will drop it down to 89

No Engine lights on

I've only had it a month

58,000 miles on it

I have the Carfax somewhere, nothing stood out as being wrong or shocking outside of a joint needing replaced, but that went bad because the guy put a set of really cheap ventilated brakes on that gave the car a terrible shimmy when you stopped.

I have a friend who works at a Chrysler dealership so maybe I will take it in and get the plugs checked. I have noticed that if i hold the RPM's in park around 1100, every 2-3 seconds it will rev itself up to 1300 and drop right back without me moving the pedal at all.

I'm not going to lie, I am not the most mechanically inclined so I don't know how to check if the prev owner had a tune installed or not, would hooking it up to a computer be able to tell me?
 
#10 ·
I looked into the service records, I don't think he ever got the 30k plugs changed and I am now at almost 60 k anyway so I will get them changed asap. Hopefully that helps. You guys are awesome btw, thanks for the help. I'll keep ya posted and post a couple pics as soon as I can give her a good bath.
 
#11 ·
16 plugs, and it won't be inexpensive to have a dealer do it.

Don't use anything but the Champion copper plugs... They work the best.

Consider doing it yourself, it isn't that hard.
 
#12 ·
agreeed: Change your plugs and service (clean) your K&N air intake filter
 
#13 ·
Also check your tire pressures. Some cars are sensitive to the pressure being low, the result being worse mileage. I'm not saying blow them up like balloons, but if your tires are at 25 psi, try 28-30 (for example). I had 255/45/20's on my SXT/AWD, and they ran best at about 38-40 psi.

If the ride feels too harsh and the steering seems very light, then the pressures are too high. If the ride feels "mushy" and the steering feels "heavy", they're probably too low. It will take some testing to find what works/feels best for you.
 
#14 ·
Ok thanks guys, yeah I confirmed that they have never been changed so I am having them done for my friends cost which is like 75 for plugs and about the same for labor so I'll prob just let them do it. I'll keep ya posted, and im also going to check my tire pressure because the steering does feel a bit heavy.
 
#15 ·
I've got a new 2011 AWD, so it isn't apples to apples, but 10 MPG sounds very wrong unless you are driving around in second gear! I agree with the others who say check the plugs and air filter. I don't recall what I got in my 2006 Daytona, but it was better than that, I'm sure.
 
#16 ·
All good suggestions here. I was wondering if you were manually calculating the MPG or just going by the EVIC reading? Could it be possible the EVIC is off? Mine measures lower than what I manually calculate by about 1 mpg, but perhaps something is very wrong with yours? Also, have you reset the EVIC reading?
 
#18 ·
It can be reset. The steering control buttons, top left inside button.

Also you can go into the extended EVIC by holding the two bottom, inner buttons until the screen changes... Go to the economy screen, it shows actual, real time mileage readings.

Does your car idle a lot? ...warm up in the morning...etc.
 
#19 ·
HA! never mind guys, mystery solved. I contacted the original owner and it does have a programmer on it, set to optimal HP obviously. It has factory set, and a fuel saver mode so i will try both to see what I like better but he is selling it to me Friday. I did find out that he put champion platinum plugs in it at 25,000 miles. I know most people including my friends say only use copper. Wondering why and if I should change them back to copper?
 
#20 ·
I changed out my original plugs to platinum at 36k. No difference in performance, just longer plug life. My car is almost at 100k now and still on the platinum plugs. I'll be changing them at 100k for a new set of platinums. My 2007 Sublime Daytona R/T has been consistently getting 24mpg highway - 16mpg city. I can get 18mpg if the traffic is light.

Also, I've noticed that the Hemi 5.7 runs about 10% better in the winter than the summer, because I live in Florida (moist 92 degree summer, dry 72 degree winter). But the gas mileage seems to stay the same all-year-round (maybe slightly better in winter, but we're talking maybe 1-2 mpg better).
 
#21 ·
P.S. - MDS will get you an extra 1-3 mpg (I get the 24-25mpg on the highway w/ MDS). I figure that in stop-and-go traffic MDS is hardly ever on, so I use AutoStick instead. But on long highway trips MDS is the way to go.

If you are in AutoStick all the time your mpg will probably be lower by 2-3mpg because you aren't getting the benefits of MDS. MDS will not engage unless you are in full automatic ('drive').
 
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