How to improve gas milage on 1994 Mazda B2300

My truck has been getting very bad gas milage. I have tried injection cleaners and a tune up. The truck is getting 13 miles per gallon; that is not good for that kind of truck. I have had it dianosed three times and still can't find the problem.How many miles per gallon should it get and what can I do to improve?

Asked by for the 1994 Mazda B2300
Is there ever a Check Engine Light on? How well does it run? How are the tune-up parts i.e plugs, wires, filters etc? What kind of diagnosis was done? Was the computer run through ALL of its self-tests? Fords/Mazdas have a very good self -test system but not a lot of people really know maximize its potential with out proper schooling. let me know and Good Luck!