In the U.S. the rate at which an engine consumes fuel is often expressed in gallons per horsepower hour. In the rest of the world, where the metric system is more common, grams of fuel per kilowatt hour is the preferred measure. Converting between the U.S. and metric systems is a multi-stage process, and you need to discover the density of the fuel in question, but the math involved is basic and straightforward.

Divide the mass of the fuel by its density to determine the volume, in cubic centimeters. For example, the typical specific gravity of diesel fuel is 0.85g, so 1,700g of diesel has a volume of 2,000 cubic centimeters -- 1,700 divided by 0.85 equals 2,000. The result is cubic centimeters per kilowatt hour.

Divide the number of cubic centimeters by 3,785, the number of cubic centimeters in a gallon. The result is gallons per kWh. For example, 2,000 divided by 3,785 equals 0.528, so 2,000 cubic centimeters per kWh is equivalent to 0.528 gallons per kWh.

Divide the value in gallons obtained in Step 2 by 1.341, the number of horsepower hours equivalent to 1 kWh. The result is gallons per horsepower hour. To conclude, 0.528 divided by 1.341 equals 0.393, so the example result is 0.393 gallons per horsepower hour.

#### Tip

University engineering extension programs often provide data covering the specific gravity of commonly available fuels.

#### Warning

Check all your calculations before trusting the final converted value.

Printed values for the density, or specific gravity, of fuels are estimates. For precise calculations you must establish the density of the specific fuel sample used.