NIST notes that it is important to consider which parts of a vehicle”s fuel tank are used to determine its capacity rating and what happens to these components when operating and fueling a vehicle. A small area at the bottom of a full tank is considered unusable because the fuel pump cannot reach that level to draw fuel. In addition, the tank”s rated capacity does not include the vapor head space, the uppermost portion of the tank compartment, nor does it include the volume of the filler pipe where fuel enters the vehicle. Drivers, however, sometimes fill the tank beyond the pump”s automatic shutoff point, resulting in fuel being drawn into the vehicle”s vapor recovery system or filler pipe. Similarly, if the lanes that surround the service station pumps are not level, fuel can shift into the vapor space allowing more fuel to be delivered into the tank. NIST cautions against using the half-full reading on the fuel gauge to determine the exact number of gallons it will take to fill the tank. The fuel gauge is intended as an approximate indication of the fuel level. Manufacturers may set the "full" indicator at a level just below the tank”s actual capacity. Reserve fuel also can be present if the manufacturer designs the fuel gauge to indicate empty at a level above the actual point where the tank runs out of gas. Abstracted from "Fuel Tank Capacity and Gas Pump Accuracy," August 2005, Weights and Measures Quarterly newsletter. Available at ts.nist.gov/ts/htdocs/230/235/archive/B-014.pdf.
