Low gas mileage can also be caused by worn ball joints and a front end that is badly out of alignment. Not uncommon for a truck with 137,000 miles.

Check the basics, such as:

new fuel filter
new air cleaner
spark plugs
clean the contacts on the inside of the distributor cap.
make sure spark plug wires are clean and not arching
set ignition timing
change the motor oil and the filter (good for 1 to 2 mpg for first 500 miles)