How to improve my gas mileage!!!
1987 Dodge D150, 3.7L, 225 SLANT SIX, 904 Torqueflite Trans, 3.23 gear 8 3/4".
Get a full tune-up, new spark plugs, over-inflate the tires, get new lower rolling resistance tires (not sure they actually make those for trucks, though), make sure your thermostat still works correctly and that the coolant is full, windows up at speeds >45 mph, drive gently, don’t haul anything around with it…I mean you’re looking at a truck that’s 31 years old, you’re only going to be able to get so much mpg improvement!!! 
Sell it and get a higher MPG vehicle. There is nothing you can do beyond the things @pyrolord314 listed that is going to make any appreciable improvement.