Fuel Economy of Gasoline Vehicles

Measuring Fuel Economy

Some drivers check the fuel economy of their vehicle once in a while; others, every time they refuel. In either case, they fill the tank, note the odometer reading, drive until they need to refuel, refill the tank, and note the volume of fuel purchased and the new odometer reading.

Fuel economy (FE) in miles per gallon (mpg) is obtained by substituting the information into this formula.

FE (mpg)  =  New odometer reading - Old odometer reading 
             ———————————————————————————————————————————
                 Gallons of gasoline to refill tank

This is an easy way to monitor one aspect of vehicle performance and operating cost. But it has its limitations as a way to detect a change in the fuel economy of gasoline.

  • First, detecting a change requires a reference point, which means that the driver must measure fuel economy regularly. It's too late to start when a suspect gasoline is in the vehicle's tank.
  • Second, gasoline variations, in most cases, change fuel economy by no more than plus or minus three (±3) percent. Detecting a change of this size with one measurement requires determining the volume of gasoline to within a few tenths of a gallon for a 15-gallon fill-up. Although gasoline dispensers are accurate to better than ±0.5 percent, a driver has no way to know if the gasoline level was brought to the same point in the filler spout or if vapor bubbles were trapped against the top of the tank.
  • Finally, as will be explained later, many other variables beside gasoline affect fuel economy. It is next to impossible to keep them all constant.

Averaging results over several months eliminates fill-up and other uncontrolled short-term variations. However, it increases the possibility that the results could be affected by mechanical changes in the vehicle or by seasonal changes in the weather.

Small Changes Can Have a Big Impact

While a fuel economy change of a few percent is hard to measure, Chevron recognizes that even small changes are important. A 1 percent increase in fuel economy would have reduced gasoline consumption in the U.S. by 1.2 billion gallons in 1997.

Measuring Fuel Economy in a Laboratory

When the fuel economy of a specific gasoline is measured in a laboratory, all other variables are held constant. The measurement steps are:

  1. Inspecting the vehicle and repairing any mechanical defects.
  2. Mounting the vehicle on a chassis dynamometer – the vehicular equivalent of a treadmill – in a building with a controlled environment.
  3. "Driving" the vehicle through a specified cycle.
  4. Measuring the weight or volume of gasoline consumed, or, because fuel economy and vehicle emissions are often determined together, calculating it from the amount of carbon compounds in the vehicle's exhaust.

The driving cycle may consist of a period of constant speed, or, to better reflect how vehicles actually are driven, may include starts, stops, idles, accelerations, and decelerations.

The cycle choice is important. Both fuel economy and vehicle emissions are very dependent on how the vehicle is operated. The U.S. Environmental Protection Agency (EPA) has designated two driving cycles for emission testing: the Federal Test Procedure Cycle (FTP 75) and the Highway Fuel Economy Test Cycle (HWFET).

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