First off, how are you calculating your gas mileage? Are you filling the tank FULL, recording the odometer reading at that time, then driving until the tank is say 1/4 full, and refilling again to the top, again recording the mileage?
IF so, then it's a simple matter to then take the difference in the two odometer readings (the mileage you drove on that tank of gas), and divide it by the number of gallons you put in the tank to fill it THE SECOND TIME you filled it.
If you are doing all of that, and you have confirmed you are getting 10MPG, then here is some stuff to check:
1. clean air filter
2. good spark plugs, distributor cap/rotor, spark plug wires
3. tires filled with air to proper pressure
4. operating temperature--does the engine run at about 200 degrees F when warmed up, or does it run much cooler?
5. how much junk are you carrying around in the vehicle. (That cord of wood you have been meaning to unload from the back for the past month probably adds 500 lbs to the gross vehicle weight!
)
6. how's your driving style? slow starts, coasting, waiting to use brakes until really necessary?
7. how fast do you drive? A roof rack with KC daylighters and a big spare tire create a lot of wind resistance at 75mph!
8. what kind of gear ratios are in your vehicle (assuming they are other than stock from the factory)?
Check this stuff out, and if you are still seeing 10mpg, report back and maybe we can give some more suggestions.