Driver for mileage

1997 Ram 1/2 4wd gets 18 combo driving consistantly.
It picks up a few mpg on highway if I keep it under 65. Anything over 65 consistantly, and the rpm, even with the od, are up, so anything 65-80 gets 15 mpg.
K&N, Headers, Magnaflow converter, magnaflow exhaust, Taylor wires and fresh tune up.

It runs, no matter what, and I cannot justify monthly payments. Unless I find a total "beater with a heater" the money I would save in fuel, would be blown on payment and then some, plus more registration fees, property tax, and insurance.