Driving habits, as mentioned before, are literally key to make things work in your favor with regards to fuel mileage. I'm not just talking about how one throttles the fuel pedal but how one also brakes. I think braking is probably the biggest thing one can alter in order to enhance fuel economy. All that energy is wasted when we brake because we are not driving hybrids with regenerative braking. Save your brakes while saving that expensive fuel in your tank.
Learning to read lights and simply letting off the throttle before braking is a big thing. Sometimes a light turns green, people start moving, and by the time I'm a few car lengths from the car in front of me I may be doing 10mph,20mph,30mph, etc and then get up to speed again. If you are consistently doing 0-45pmh all the time, your wasting fuel. Some English study I read awhile back showed how moving at 10mph constantly is better than stopping and restarting beyond 10mph. I can't source the article at this time but overall it makes sense. I rather be crawling in traffic at 5mph then running through the gears on and off for an hour.
My girlfriend lives in a town with a lot of train tracks. We always hit a train!
These are typically not just commuter trains but rather trains that crawl with like 200+ cars on them. Honestly when this happens I just put it in park and shut the engine down... yes even in the winter. Idling is by far the worst form of fuel consumption. When your parked you are getting ZERO MILES PER GALLON
. Do the math. Your not moving and your burning fuel thus no miles traveled.
Also, warming your car up in the winter is pointless as well. Especially with these Jeeps! My XJ is up to temperature in less than three minutes of normal driving. http://www.consumerenergycenter.org/myths/idling.html
Aerodynamics are key and it shows in fuel mileage tests time and time again. The hand of god (I.E. Physics) will always win! For example, just look at the new Jeeps. If all things were equal, a two door TJ would consume more fuel than a two door JK.
The fact that Jeeps traditional grill has seven slots in it really doesn't help aerodynamics at all. While this is great for low speed trail speeds, it hampers the aerodynamics of the vehicle itself at highway speeds. Air at highway speeds is sucked into the front instead of going around the vehicle. Blocking the holes might be a good idea in the winter months but your Jeep might stay hotter for a longer period of time in the summer months.
Another thing that might seem odd is the antenna is huge. Just look at it at 45+ miles per hour. The thing is wiped back. Shortening it will reduce a small amount of drag on the vehicle while making it look a littler more stylish. Yes your not going to be getting even just one more mpg out of this mod but every little bit helps.
Keeping it at 65mph on the highway is the optimal speed for most vehicles regarding fuel mileage. After that aerodynamics works more against you than with you. One way I literally have fought this is drafting off of semi trucks through the bulk of my destination. I typically do this going to and from my moms house across the state and this trip is roughly 140 miles one way. When I get behind the semis I usually pick out ones that are traveling faster than 55pmh. You can totally tell the wind noise is reduced around you, and at the end of my trip, I get roughly 20-30 miles more per tank. That was on my old car however, so I can't speak for my Jeep. Regardless, I'm sure I will be seeing an increase in mileage per tank following this same process.
I always swear by a TRUE cold air intake (CAI) and a decent cat back exhaust. While it doesn't give you more mileage and performance by adding these items it does help give you more mileage and performance by reducing the restrictions these stock items put on your engine. Properly tuning the ECM will help out as well but for cost sake I will omit that option. This items will need to pay for themselves and that honestly could take years of miles driven before any cost return would be seen. Personally, I would just do it for the sound and performance aspect and have the increase in mileage as an added bonus.
Lastly, and probably most importantly, O2 sensors are the biggest thing to check with regards to keeping your fuel mileage consistent. When they go bad the engine typically reads lean and dumps more fuel into the engine. I always check them when I buy a new vehicle. If they are thickly covered in carbon then I replace them. I remember on my Nissan Altima I received a 2mph increase from just changing out the after cat sensor (the back one). If the sensor isn't working right, you will notice it in your fuel mileage. When it comes to maintenance of these sensors I stick to 5 years or 60,000 miles, which ever comes first to change them out.
I think its fair to say things like tires profile,maintained tire pressures, and routine maintenance will really keep your fuel mileage optimal. The only real way to increase fuel mileage is to dump the current motor and go diesel.
However, since that isn't in most peoples budget, including me, we can only monitor our driving, pay attention to the aerodynamics of the vehicle, and optimize our vehicle maintenance.
Hope this helps, sorry if I repeated anything. I read most posts before I put this up.