Grams per kilowatt-hour, abbreviated g/kWh, provides a measurement of how much fuel a car engine consumes per unit of energy it produces. Though you may not see a connection between this measurement and the speed you drive in miles per hour (MPH), you can calculate grams per kilowatt-hour if you know the number of miles per gallon (MPG) your car gets at that speed and the horsepower rating of your engine.
Divide the MPH by miles per gallon to determine the amount of gasoline your car consumes in an hour while driving at this speed. If you drive 60 MPH and your car gets 30 MPG, for example, 60 MPH/30 MPG = 2 gallons per hour.
Multiply the number of gallons consumed per hour by the number of hours you drove the car to get the total number of gallons used. If you drove the car for 1.5 hours, for example, (2 g/hr)*(1.5 hr) = 3 gallons.
Multiply the number of gallons used by 0.134 cubic feet per gallon to find the volume of gasoline you used in cubic feet. For example, (3 gal.) times (0.134 ft^3/gal.) = 0.402 cubic feet.
Multiply the number of cubic feet by the specific weight of gasoline, 42.5 pounds per cubic foot, to get the weight of the gasoline used. For example, (0.402 ft^3) times (42.5 lb/ft^3) = 17.1 pounds of gasoline.
Multiply the horsepower of your car by the number of hours you drove to get the horsepower-hours (HPH) of energy consumed by the car during that time. If your car produces 150 horsepower (HP) and you drove for 1.5 hours, for example, 150 HP times 1.5 hr = 225 HPH.
Divide the pounds of gasoline used by the horsepower-hours of energy produced to find the car's fuel consumption in pounds per horsepower-hour. For example, 17.1 lb/225 HPH = 0.076 lb/HPH.
Multiply the number of pounds per horsepower-hour by 616.78 to determine fuel consumption in grams per kilowatt-hour. For example, (0.076 lb/HPH) times [(616.78 g/kWh)/(lb/HPH)] = 46.9 g/kWh.
