Fuel Managment.. NEED HELP!!!!!

I have had my Truck for about 4 yrs.. the first 2 yrs i had it i got like 400 miles to a tank.. now i get 260 mile to a tank.. i would like a good run down list of everything that could be wrong with it.. i have replaced so many parts and done so much and i cant figure it out..If anyone has had this problem or knows what to do let me know...

Have you done a compression test? If it's good then a good tune up, new air filter, plugs, and what not. Check the plug gap... it's so often over looked.

I saw an article in Hot Rod mag a few years ago, about spark plug gap, vs fuel milage, vs power that would probably intrest everyone. I'll try and find it, and post it here soon.

have done a turn up on it.. um i pretty much change the Air filter, plugs and wires every 3,000 miles when i do my oil.. i had to change the cat and a week later the problem started do you think the cat might be the problems..

if you changed your cats you are going to want to reset the ecm memory. pull the ecm fuse/relay for ~5-10 minutes. that will clear all the memory in the computer. your vehicle will run crappy for a few minutes until it can relearn all the sensor values. i would recommend changing all the o2 sensors. over time there not as efficient causing fuel mileage loss. a compression test will tell you if your engine is mechanically sound. low compression will cause poor fuel economy. i would check the fuel system. i am assuming this vehicle has the vortex motor with the spider injection system. check the valve for leakage. if it is leaking, it will cause a rich condition. check all that and get back to us.

You change plugs and wires every 3,000mi.???????