I love to tinker, so I've tried many things, both with my Hemi LX and my Hemi WK. So far I've gotten the LX up to a high of 31 mpg and WK up to a best of just over 22 mpg on the highway. Both get about the same (LX slightly higher) in town and mixed....~14-16 mpg in town and ~15-17.5 mixed. The only things that I have found that make significant differences are a good, complete (including tubular headers) low restiction exhaust system (worth almost 3 mpg on the Hemi), reducing drag coeficient by lowering, and adding SRT bumpers and vortex generators (worth about 1 mpg.), and driving at the engine's "sweet spot". On the Hemi's that is ~1800-2000 RPM. Other than that an accumulation of little things may help slightly. Here are the things I've done to both my Hemi vehicles. Same probably would apply to the 3.7 and 4.7 engined WK's too.
1. Have the vehicle aligned in the configuration it is most driven in....i.e we have both ours aligned with the wife and I sitting in them, as this is the way it is driven most of the time. Alignment will change (sometimes to out of tolerance) once weight is added. This tip came from my alignment guy.
2. Run tire pressures on the high side of what gives you the most even tire wear. I run all mine at 38-40 PSI fronts and 36-38 PSI rears. I also run nitrogen in all my tires and they are all siped http://sipers.com/
. Both of these things are somewhat controversial, but if it helps a fraction, that is good, IMO.
3. Unless you use the roof rack cross bars all the time, remove them. This will help a little by reducing wind drag. This also goes for cargo racks, spare tires, etc.
4. Take a look at the front end of the WK...it's like pushing a box down the highway. I added SRT8 bumpers, and Airtab vortex generators. http://www.airtab.com/
Be sure to watch the video indicated in the box at the bottom of the page. It does work, but is hard to quantify due to so many variables. This also will keep your rear hatch window completely clean on the highway....guaranteed! They don't work in town.....only above~40-45 mph. I will also be adding VG's to the underside of the WK. I have them under the LX, on the skid plate, in front of the rear suspension and under the spare tire well to keep the air turbulence from curling back onto the rear bumper (See Airtab video). Before I added the SRT bumpers, I chopped about two inches off the lower air dam in an effort to try and reduce frontal area.
5. I have electric folding mirrors on my WK and fold them back when on the highway. Stick your flat hand out the window at 60-70 mph and see how much drag resistance is on your hand....and your hand is probably a little smaller than the area of each mirror.
6. Drive at the "sweet spot" for your engine. As I said, mine is at ~1800-2000 RPM for my Hemi's. That equates to ~58-60 mph in 4th or ~63-65 mph in 5th. The faster you drive, the lower your mileage will be, especially with the smaller, lowered powered engines. HP and torque requirements (more throttle) increase with the speed, and economy starts tanking badly.
7. If you can get a tuner (any brand) that adds light throttle timing, buy it and use it. More timing WITHOUT creating Knock Retard will improve economy. If you get the Superchips Tuner, be sure to get the Advanced Transmission Tuning Application, if you have the 4.7 or 5.7 engines.. This will allow you modify the lousy shift schedules on the 545RFE transmission for you guys with the 4.7's and 5.7's. Since the 3.7's and 6.1's use the NAG1 (I wish the 4.7's and 5.7's had this too....I have it in my LX), you don't need it and it's not available.
8. Do a complete (including tubular headers) low restriction, high performance exhaust system, if possible. I don't believe tubular headers are available for any of the WK's except the 6.1. These headers can be retrofitted onto 5.7's, but not without some problems. I had my 5.7 headers custom built for the WK. I also have seen on these forums that some folks are experimenting with retrofitting headers from similar vehicles.
9. Add a low restriction air filter element, such as a K and N or True Flow. This will not make much difference, especially in town. It will help most at WOT, and slightly on the highway.....better if you have the exhaust though. As a rule, IMO, aftermarket CAI's are only good if you want more noise or are going racing. The stock engine intake systems are more than capable of handling all the air the engine is capable of ingesting. Be sure your intake system (whatever you use) is low/no restriction. That means as smooth (no restrictions or obstructions) on the inside of the intake tract. Also, if it's not already, try to have a tapered system. For instance, on my LX and WK I have two air inlets into the SRT8 airboxes, and a 5 inch exit tube that tapers down to the ~3.25 inch throttle body butterfly. This helps to slightly increase (venturi effect) the air velocity as it enters the intake manifold.
10. Use synthetic oils/fluids EVERYWHERE......differentials, transfer cases, transmissions, engines.
11. If you have more than ~60K on your vehicle, you may want to consider replacing the oxygen sensors, especially if your fuel economy is worse than normal/expected. I had one bad and one marginal at the 60K inspection/maintenance and no lights indicating a problem. I replaced them all for about $120. See my thread: http://jeepgarage.org/showthread.php?t=20300
12. If possible, remove all unnecessary, excess weight from the vehicle. You know if you have it.
13. Run the lowest possible octane fuel possible for your engine. Using a higher octane fuel than needed is just a waste of money, and your economy will probably be adversely affected slightly. If you are at altitudes above approximately 2500 ft., you can usually use a lower octane fuel than normal. The rule of thumb is one octane number lower for every thousand feet of elevation. I am at ~2800 ft. and run 87 in both my Hemi's. When I travel to higher elevations (above ~4000 ft.) I run 85 octane, without issue. See my thread: http://jeepgarage.org/showthread.php?t=22843
My next (and probably last) "serious" mod. to both my Hemi's will be to change cams to one that has been proven to increase power, performance and MDS usage. If you can't get a better cam, you may want to consider advancing the stock cam by ~2-4 degrees. This will give somewhat better low end torque and power and, as a result, should improve economy a little.