Dwindling MPG's? Ain't nobody got time for that!

Hack12

New member
I have a 2008 JKU Automatic with 70,000 miles on it and it seems to be dropping MPG's faster than a drunk girl in stilettos.:doh:

Here are some fun facts that should answer the majority of your basic questions. All my fluids are up to date and have always been changed at the scheduled intervals. Like most other 3.8L's, it burns oil like a mother trucker, approximately 1qt per 1k. Unsurprisingly, when I mounted my 33x12.50x18 Mud Grapplers and RR XHD bumpers I lost a couple MPG's. Conversely, when I ditched the hardtop and installed the Trektop NX I gained about 1 MPG. I am getting no idiot lights and the engine runs and sounds the same as it always has.

Over the past year, and after all the aforementioned modifications were installed, I began watching my MPG's drop at a pretty consistent rate. I began 2013 with my dash panel display showing approximately 13-14 MPG's, depending on driving conditions, and will be ending the year at 10.5 MPG's!

Is this common for a 6 year old 3.8L with seemingly no other issues?

I am stumped.:thinking:
 

JandSJK

New member
That's fun you should mention this. I have the same rig about the same miles. I have been running the same basic setup since March. My mpg has dropped slightly. Was thinking about doing a fuel system flush and new plug and seeing if that helped

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RockyJk

Active Member
Did u recalibrate your speedo after you put on the tires?? The numbers its showing might be off if you haven't
 

Hack12

New member
Did u recalibrate your speedo after you put on the tires?? The numbers its showing might be off if you haven't

I didn't recalibrate the speedo, but the tires have been on for over a year and the MPG's has been getting progressively worse during that entire time. The last couple months have been the worst, dropping nearly 2 MPG's in that time. I can't imagine a speedo recalibration would explain the slow decrease over such a lengthy a period of time. I'm more concerned with finding out why it's steadily dropping than I am about the actual dismal MPG's and whether the ECU is calculating them correctly.
 

BigBoy

New member
Have you switched brands of fuel lately? I find it pretty common to see a decrease in the winter of about 1 mpg or so from the fuel, but I always use the same brand fuel.
 

Hack12

New member
Have you switched brands of fuel lately? I find it pretty common to see a decrease in the winter of about 1 mpg or so from the fuel, but I always use the same brand fuel.

I usually fill up at Shell, and occasionally a few Mom & Pop places. I had the same thought about winter fuel too. I've just never seen it go this low before. High 11's has been as low as I've seen it dip in the 3 years I've owned it.
 

Hack12

New member
Did you add the 33's this year? How is the air pressure?

Other than that, I'm stumped too.

I added the 33's in November 2012. Air pressure has been a couple pounds higher than normal for the past few weeks. Think that could be the reason for the recent negative slide?
 

jeeeep

Hooked
not uncommon, as your tires wear and the pattern changes you will lose mpg's even more so with mud tires.
 

Hack12

New member
When is the last time you did a tune up? Spark plugs, air filter, fuel filter etc?

Plugs and wires were replaced at about 60k, air filter is a K&N CAI that hasn't been cleaned since I installed it over a year ago, and I'm not sure I've ever changed the fuel filter.
 

H8MUD

New member
I would disconnect the battery, touch the negative and positive and leave it disconnected for 10 minutes. Then reconnect and see if that helps. Had a similar problem and that fixed it.
 

Hack12

New member
I would disconnect the battery, touch the negative and positive and leave it disconnected for 10 minutes. Then reconnect and see if that helps. Had a similar problem and that fixed it.

That'd be great, quick and easy. Certainly worth a shot!
 

jeeeep

Hooked
K&N's website states that their filters only require cleaning and oiling every 50,000 miles. I only have 15-20k on mine so I figured I was alright.

depends on where you drive, one day in dusty moab and it's time to clean it! (actually I carry a spare to put on for the drive home when I go to EJS because it is so dusty there)

you should give it a look
 

Hack12

New member
depends on where you drive, one day in dusty moab and it's time to clean it! (actually I carry a spare to put on for the drive home when I go to EJS because it is so dusty there)

you should give it a look

Yeah, I'll bet it's dirty as sin after not cleaning it for so long.
 

Hack12

New member
A little update, I went ahead and changed out the cone filter on my K&N CAI. In a matter of 2 days my average MPG's went from 10.2 to 14.8, and may still be climbing!
 
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