any suggestions?
 

well, my truck seems to have settled on about 5.3 miles to a gallon, it doesnt matter if I go 55 or 65 or 75, it gets about the same. Any suggestons on what I might do? I know I need to get it put on a computer but other than that, what should I do?
it's a C-15 cat says 435 on the tag, but I dont know if it's been turned up. has 13 speed and 3.55 rears.

You might want to put some injector cleaner in your tanks. If that doesn't help then I would take it to a shop and have them do a diagnostic on it. The cost should not be that great. It could be a number of things. It might be a bad injector. A friend of mine had one and it took a long time to find it. Most times you can tell if you have a bad injector, but if they are not firing properly they can be difficult to diagnose. I would take it to a CAT dealer and just have them put it on a computer if the injector cleaner doesn't work. I use transmission fluid in my fuel to clean my injectors. It is an inexpensive way to clean them. They make several different types of commercial injector cleaner. I know you don't want to spend the money, but you are burning a lot of fuel. If you could only increase your fuel mileage by as little as 1/2 mpg it will pay for the diagnostics in a very short time.

I dont mind spending the money to get it right, I just dont want to waste a lot of money, I am going to change the filters today, I did put a big bottle of lucas in each tank and it didnt help. One time, I got 6.9 but that was only once and I didnt do a thing different, so that's what leads me to believe it may be in the computer, I am making good money, but spending it all on fuel, I guess to the cat dealer we go. like you said, even a little increase will make a big difference.

Yancy Cat in Atlanta Fre...Exit 51 on the Loop. Decent shop.

Dirty air filters could cause your engine to consume more fuel.

one of the first things I checked, it was serviced when I bought it. oil change, filter, fuel filters and new air filters. I looked at them today and they are clean, I also checked the intake pipe and it is clean.
maybe spark plugs, points rotor and condensor would help. and new jets in the carb too.

How many miles on it? Maybe it needs the "Rack" run??

It's a W9....so not going to get stellar mileage...but you should be able to get 6, maybe 6.3 mpg.

530,000 the rack is on my list of things to ask cat about.

I have a detroit engine that gets about 5.2mpg regardless of everything I do to it. I did overhead on injectors,change filters,put it on computer and still nothing. It has 1 million 100k miles on it. I try to go slow from 110mph to 60mph,get passed by mean old people and still fuel mileage same. WHY? Is it time for new truck,yes?

I thought you were going to tell me to try farting in the fuel tanks.