Use your vehicles tripometer to measure trip distance. (Remember to reset the tripometer at every fill up.) Print receipt for gallons refuled. Input trip distance from your tripometer and the # of gallons purchased from your receipt. Press calculate to know your mpg averaged over trip duration.
Gas mileage can be improved by many factors including tire pressure, driving style, clean air filters, and reducing weight.
More specifically your car's average mpg can benefit from removing excess weight and drag, such as ski or bike racks, and luggage racks. Inflate tires to recommended levels, and replace air filters regularly or sooner if driving in dusty conditions. Optimize your car's gearing at a lower rpm, the engines demand for fuel will be less. Travel certain roads or at certain times to avoid stop & go congestion/traffic lights. Fill your tank with less gas next time at the pump to save on weight.
For the hardcore enthusiast, reducing weight can also improve acceleration, deceleration, and handling in addition to gas mileage. Consider removing your rear seats, air conditioner, the spare tire, and replacing body panels with lighter but durable carbon fiber material. Consider replacing stock parts like rims & batteries with lighter alternatives. If you are replacing with aftermarket parts consider the weight as well as performance to knock out two birds with one stone.