I have an 04 gp gt with 160k and like you I'm concerned with with my gas mileage as well
HERE is what you need to do:
Maintenance:
-new spark plugs
-oil change
-clean your MAF - ONLY with MAF cleaner
-clean the inside of your throttle body - TB cleaner or engine degreaser
-clean the inside of your upper and lower intake manifolds - engine degreaser I can guarantee that with your mileage the inside of your tb and intake manifolds are covered with **** and gunk and need a good cleaning
-seafoam
-keep your tire psi on the higher side (I keep mine at 40 and my tires max are 45/50)
Mods:
-remove the plastic insert in your upper intake manifold - FREE (this will also give you a small power gain in the upper RPM range)
-make your own HAI with a cone filter - $40
-get a DP or headers - $80-180 (I haven't done this yet but I've heard it will net you gains as well)
-get a tune or canned tune
Driving Habits:
-stay below 75 on the highway - the higher you get over 70 your mileage will start to drop (I try to stay at 70-72 if the speed limit is 70 and lower for lower speed limits, your best mpg will be in the 55-65 range)
-during highway driving try to stay behind another car, less air resistance -> engine does less work
-DO NOT accel at a turtles pace, you want to minimize the amount of accelerating you do and maximize the amount of cruising - even when you are accelerating slowly your mileage is cut by about 60-70% of what it is while your cruising (when going from a stop, I stay between 2500-3000 rpm until 40 when OD kicks in)
-cruise at a constant velocity as much as possible - the majority of your fuel is used during accel, the less accelerating you do the more gas you will save, this is easy physics, it takes a LOT more energy to get 3600 lbs to accelerate than it does to keep 3600 lbs at a constant velocity. so try not to bobble up and down between 40-45 mph because it will reflect in your mileage
-stay aware of your surroundings - if you see that the light is turning red ahead, let off the gas. or if you see a red light ahead and know it will be turning green soon try to pace yourself so that you will be able to maintain atleast some of your speed when the light turns green so that you dont have to use more gas by coming to a stop then accelerating back up to 40
Last week before taking a long drive, I cleaned my upper intake manifold, removed the plastic insert, clean my MAF and throttle body (again just because I was going to have them off anyways), and seafoamed my car. This had a BIG effect on my gas mileage, it also felt like it gained a bit of power in the upper RPMs because I removed the plastic insert. Normally when I fill my tank up before a long highway drive my gas needle will start to fall from the full positions at 35-40 miles into the trip, but after I did all of this, I was amazed to see that the needle was just starting to fall at the 50-55 mile mark, it wouldve been in the 60+ range if i hadnt done a couple of WOT runs to check my power gains from removing the intake insert. My rpms were also about 50=100 revs lower while cruising at 73ish. And city driving seems to have had a bigger improvement than that. I attribute most of the gains to removing the upper intake insert and cleaning the intake manifold because it freed up a lot of restriction. Hopefully when I get a DP I will net a few more mpg's
Hope this helps
