Between the tire size and whatever gearing you are running may be a big part of it.
The gearing may be loading the motor up too much and not letting it stay in the power range it was intended. Like someone else said early, if you are not correcting for the difference in actual mileage from the speedo, then that will definitely change things.

A good old tune up will usually help any engine, unless you have recently done this to it. Just some ideas. Good luck.


P.S. My truck gets between 17.5 and 19 MPG depending on how big of a hurry I am in. It has 275-60-15 tires on it. So when I am just cruising along, it gets pretty decent for a truck, BUT when I get in a hurry, it likes to drink the gas.