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Is the "check engine" light on?
Have you tried a code reader? Are there any codes pending?
If there are no codes showing/pending (because you must remember that auto manufacturers have attempted to make their vehicles "idiot proof", it's far cheaper to charge the customer for electronic mumbo jumbo, than train mechanics) you must step into the basics. Thermostats in Ford trucks (Rangers) seem to last only a year, or two, replace your thermostat with one of the proper temperature (this is the first step, and the cheapest, in trying to regain mileage), next take a compression test of all the cylinders to check on the engine's general condition. I happen to know that sometimes the gasket between the intake manifold and the plenum chamber takes a shit, not really detectable with a vacuum gauge, and sometimes spraying oil on the seam does nothing (because the part of the gasket that takes a shit is at the back, where you can't get at it properly). A real bitch to get off, if everything else looks ok, it is worth a look.
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