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 Post subject: 2007 WK CRD Poor Gas Mileage
PostPosted: Mon Nov 28, 2011 12:33 pm 
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A bit of background info first: I have always owned cars, but I have been looking for a truck to use as a family vacation vehicle and to haul my motorcycle. I finally bought an 07 WK 3.0 CRD Limited 4x4 with ~124K miles about a month ago. I have been lurking this site a bit before and ever since. I had researched both WK and XK with the 5.7 HEMI and — from what I had read — it was close to impossible to get decent MPG. Hence, when I came across the turbodiesel, I thought I could get the best of both worlds with great torque and decent MPG.

When I first got it, the computer showed about a 17 MPG average, but I did not know of the previous owner's driving habits and thought for sure I could do better. Well, the average declined ever since until it went below 15 recently. No matter what the claimed service history of the vehicle is, I knew when I got it I would need to do injector cleaner, filters and oils (engine and tray) service to get a baseline (which I did), and thought it would improve after. It did to a certain extent, but it does not climb too far past 15.3 or so. All along i have been monitoring things with a Scan Gauge II hooked up to get real time readings, and try to find sweet spots for the engine. I have been driving it different ways: first accelerating very gradually from a standstill. I then tried to test this theory that my slow acceleration could be forcing the engine a bit (with the turbo not spooling much) therefore affecting gas mileage negatively. So I tested — as I thought the initial torque of the engine would make a difference — accelerating it "semi hard" to around 40-50 and then letting the gear box get to 5th. Both methods yielded about the same average. Still get about 15 mixed, which is a lot worse than I think it should be doing.

Now the questions:

Besides filters and regular maintenance, what other problem areas should I look for in the diesel to get the original gas mileage back? At least something close to it anyway.

I have also been looking at lifts and general mods that can be done. From people's experience, how much does that affect MPG negatively? I almost wish there was a guide of sorts: I.e. Lift = -1,2 MPG, skidplates = -2 MPG, roof rack (I want a full size one to have a tent) = -2 MPG, etc. You get the idea. Even though there probably is no such guide, I am sure I can get a general guideline from people who have done the mods.

Any other general advice on getting better MPG out of this monster?

I look forward to the responses, thanks.


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 Post subject: Re: 2007 WK CRD Poor Gas Mileage
PostPosted: Fri Dec 02, 2011 9:34 am 
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The largest factor to poor fuel economy on the WK is the axle ratio. You are spinning close to 2800rpm at 80mph, whereas the KJ CRD at the same speed is close to 2400rpm. This makes a big difference. Also, during winter you can expect about 2mpg drop due to winter fuel and the colder temps.

We have an engine tune for the WK that will help you gain about 2-4mpg on the WK, along with a smoother powerband, increased torque, reduce turbo lag and much cleaner oil (less soot loading). Good Luck.

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 Post subject: Re: 2007 WK CRD Poor Gas Mileage
PostPosted: Wed Dec 07, 2011 11:21 pm 
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Thanks for the reply. I wrote to you when I first got the WK, and have been meaning to call you, but I wanted to see if I could get closer to the expected MPG before going with a tune. I am going to call you to talk more about the details of the tune this week.


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 Post subject: Re: 2007 WK CRD Poor Gas Mileage
PostPosted: Thu Dec 15, 2011 11:13 am 
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Location: NH
The rpm numbers GDE gave you are at 80, but even at 65-70 you'll be well into the 2000 rpm range while my Liberty CRD is just hitting 2000 rpm at those speeds, maybe 2,200 at 74 mph. That diesel would like to cruise the highway at 1,800 rpm for best economy, stay out of the turbo as much as possible. I think it would still tow great with axle ratios that kept the rpm down, though I looked and the max torque happens in the mid 2000's so that's probably why it's geared that way, I think it has 3.73. I'd look at putting the largest diameter tires you can fit on it to get rpm down, but you still need light weight, low rolling resistance tires or the advantage will be lost. Ask GDE if they can compensate for the bigger tires in their tune, not sure.

I've thought recently about trading my Liberty CRD for the Grand diesel, just to get the fancier interior and a bit more room. There is one for sale nearby that I might go and test drive. But, with that kind of loss of mileage I'd have to think twice, plus I really enjoy the low rpm, smooth cruise of the Liberty on the highway.

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GDE Eco Tune Full Torque, GDE TCM Tune, Firestone Destination LE 225/75-16


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 Post subject: Re: 2007 WK CRD Poor Gas Mileage
PostPosted: Mon Dec 19, 2011 10:46 am 
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Dave01 wrote:
The rpm numbers GDE gave you are at 80, but even at 65-70 you'll be well into the 2000 rpm range while my Liberty CRD is just hitting 2000 rpm at those speeds, maybe 2,200 at 74 mph. That diesel would like to cruise the highway at 1,800 rpm for best economy, stay out of the turbo as much as possible. I think it would still tow great with axle ratios that kept the rpm down, though I looked and the max torque happens in the mid 2000's so that's probably why it's geared that way, I think it has 3.73.


From what I read it is a 3.73 for the CRD and V8. I have been monitoring RPMs and MPG with a ScanGaugeII, but I have a feeling the readings I am getting are off a bit. My theory is that since the readings on the EVIC is known to be unreliable (hand calculated MPG is a few miles off) the same goes for the readings it is getting on the SGII. I read somewhere that some of the tunes improve this and you end up with a more accurate reading on the EVIC.

Quote:
I'd look at putting the largest diameter tires you can fit on it to get rpm down, but you still need light weight, low rolling resistance tires or the advantage will be lost. Ask GDE if they can compensate for the bigger tires in their tune, not sure.


That is interesting and logical I guess. I wonder how much of again that might be. If I were to get a lift (which I want to do eventually) to fit bigger tires I am wondering if any gains will be a wash. Also, as I understand it, the gains would be only for cruising. You would strain the engine a bit more off the line, so the MPG there will suffer there, right?. There has got to be a formula to calculate MPG gains based on size of tires, but I have never come across that.

Quote:
I've thought recently about trading my Liberty CRD for the Grand diesel, just to get the fancier interior and a bit more room. There is one for sale nearby that I might go and test drive. But, with that kind of loss of mileage I'd have to think twice, plus I really enjoy the low rpm, smooth cruise of the Liberty on the highway.


I can't speak for the Liberty, but the grand is nice, and since they came as limited models they have all the trimmings: Nav, leather, backup camera, TPMS, Sat, etc. But from what I hear, the MPG difference on the two is substantial. The GC does have the top of the line Quadra Drive II in its four wheel drive, but it is permanent, and I am sure you can get some gains by running it 2WD for the highway on the Liberty.


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 Post subject: Re: 2007 WK CRD Poor Gas Mileage
PostPosted: Sun Jan 08, 2012 5:17 pm 
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Location: La Vernia, TX
When ours was stock, we were slightly over 20mpg. Coming back from SLC to Texas, I was showing 25mpg in the mountains. On the flats, it dropped down to 20-21.

Ours now has the 2.5" lift, 265/70/17 (Michelines) and with a new air filter, I was showing 18 on the EVIC. Correction for tire size puts me close to 19mpg if you use the EVIC numbers to start. I have not hand calculated it since my wife drives it most of the time. Prior to the service I was at 16.8 EVIC so the air filter was ready for replacement. I also think our BFG All Terrains had more rolling resistance than our Michelin's. Sure the BFG's look great but honestly, we never off roaded the WK so it was a waste of money IMHO. The Michelin's at load range E even ride much better. I keep them at 50-51psi.

Check the air inlet seal to make sure you're not leaking boost. If you didn't do the diff/trans/trans case service, do it. Make sure you use synthetic per the manual and add friction modifier. Running regular vs synthetic can make a difference. I would also check your wheel bearings and/or brake drag. Both can make a substantial impact on mileage.

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2005 CRD (RIP 2/5/10)

2008 Grand Cherokee Limited WK, CRD, 4x4, lift, 32's.
2008 HEMI Wrangler Unlimited Rubicon JK, lots of extras


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