Another agency with fuel economy ratings is the California Air Resources Board CARB. CARB offers a lookup site for the past 10 years of vehicles. The site lists detailed smog and global warming scores as well.
There are other car enthusiast type sites that may make and report some of their own quantities and parameters. However, the Federal government actually measures the quantities and parameters for a large number of vehicles using standard methods. As the sign sometimes says regarding vehicle mpg: “actual results may vary."
Your actual mpg will depend on the year, design, weight, maintenance, and driving style. Big heavy cars, trucks, and SUVs still get pretty lousy gas mileage. Small, light, well designed and built cars get the best mileage. However, it is important to maintain your vehicle to specification for the best mpg. Driving style is how fast you accelerate and stop…all day long. If you want to race around, it will burn a lot more gas, and put a lot more wear and tear on your rig. Take it easy for the best mpg.
Vehicle mpg is well documented by government agencies like the US EPA. You can look up just about any production car made in the past 25 years. Fuel economy ratings are given.
Interestingly average miles per gallon of cars in the US only improved from 14 mpg in 1923 to just 17.4 mpg in 2008. This according to the University of Michigan.
Alternatively, there are a few MPG champs: How about the Cal Poly "Black Widow." This car tips the scales at 96lb (44kg), and tops out at around 30mph (50kph). The Widow is powered by a tuned 3hp Honda 50 engine. The first run of the car yielded 861 mpg (366km/L). A recent run returned 2,752 mpg or 1170 km/L.
Lets compare this to an electric vehicle. Take a stock 220 lb (100kg) electric scooter.
This is an electric scooter with a 1,500 Watt (2hp) motor. The scooter uses 30 Watt Hours per mile of travel. There are about 36,600 Watt hours of energy in a gallon of gasoline (wiki).