August 2005 

Fuel Tank Capacity and Gas Pump Accuracy

By Juana Williams 

Often when fuel prices rise as they have during 2005, weights and measures officials see 

an increase in the number of complaints related to the accuracy of gasoline pumps (retail 

motor-fuel dispensers). Frequently, weights and measures jurisdictions report receiving 

consumer complaints from motorists who believe they filled their vehicle's fuel tank with 

more gasoline than the tank is specified to hold. The possibility that the pump may be 

inaccurate exists, but typically complaints arise because the vehicle tank capacity rating 

is only an approximate volume. In this article we will examine some possible reasons 

why a motorist might see variations in tank capacity indications. 

Rated vs. Actual Fuel Tank Capacity 

The vehicle's fuel tank capacity rating is a reasonable approximation given variables such 

as differences in design practices, test procedures, and other influence factors on the fuel 

system. Some manufacturers estimate that fuel tank capacity can vary by as much as 3 % 

from the actual tank capacity as a result of normal variations in design characteristics, the 

manufacturing process, and other inherent factors such as the physics associated with 

components and the electronics in the fuel system and fuel level indications. Knowing the 

vehicle's fuel tank capacity provides the driver with information needed to plan driving 

distance, refueling stops, fuel costs, etc., associated with highway travel. Tank capacity is 

also a factor used by vehicle components that monitor emissions and other on-board 

diagnostic equipment that affect fuel level indications. Typically, the owner's manual 

provides a reasonable estimate of the vehicle's fuel tank capacity. Some owners’ manuals 

list fuel tank capacity in SI (metric) units, equivalent U.S. customary units, as well as 

other international units of measurement such as the British Imperial gallon, for example, 

70 L (18.5 gal, 15.4 Imp gal). The tank capacity's numerical value is reported in tenth of 

whole figures rounded to the nearest value and, therefore, represents an approximate 

volume. 

Various related terms are used by auto manufacturers and associated agencies to describe 

the actual (liquid- and vapor-space) and advertised fuel tank capacity and fuel levels 

important to drivers. In March 2005 the Society of Automotive Engineers (SAE) 

Refueling Interface Subcommittee published SAE J398, a guide that clarifies terminology 

as well as a procedure for establishing fuel tank capacity for passenger vehicles. In SAE 

J398, the vehicle's fuel tank capacity rating is reported as an average volume based on 

test data gathered on the capacity of multiple tanks. Representative samples of a 

particular type of fuel tank are filled to a prescribed number of automatic shut-offs by the 

pump nozzle to determine fuel tank capacity. Attempts by drivers to fill the fuel tank 

beyond the pump's automatic shut-off point can result in fuel being drawn into the pump's 

or vehicle's vapor recovery systems. Auto manufacturers choose the test conditions and 

may vary the test mode based on the intended use of the vehicle. 
Drivers might find that a vehicle's fuel tank capacity rating includes only the usable 

portion of the tank's liquid capacity shown in the diagram below or both the usable and 

unusable portion of the tank's liquid capacity. 

The unusable volume is the portion of the fuel tank's liquid capacity that lies at the 

bottom of the tank out of reach of the fuel pump. The usable volume is the portion of the 

fuel tank's liquid capacity that can be delivered through the filler pipe into the tank when 

the vehicle is level (and when the unusable volume is also in the tank). The vehicle's fuel 

tank capacity does not include the vapor headspace (that portion of the tank compartment 

at a level above the filler pipe neck) or the volume of the filler pipe. Sometimes drivers 

ignore the pump nozzle's automatic shut-off and continue to hold the nozzle operating the 

lever open in an attempt to deliver additional fuel. When this happens, the additional fuel 

begins to fill the vapor headspace and the filler pipe that are not considered part of the 

tank's rated capacity, thus resulting in a delivery of fuel greater than the fuel tank capacity 

rating stated in the owner's manual. Similarly, the lanes that surround the service station 

pumps may not be level and fuel may shift into the vapor space thus allowing more fuel 

to be delivered into the tank. 

Indicated Volume 

Another scenario that can prompt consumer complaints involves the fuel tank capacity 

and the fuel gauge indication. When the fuel tank indicates a particular level, the 

customer frequently assumes that this represents a corresponding fraction of the fuel tank 

capacity. For example, on a fuel tank with an 18-gallon rating in the owner's manual, the 

consumer assumes that a "half-full" fuel gauge indication means that 9 gallons of fuel 

remains in the tank. When the consumer is able to dispense more than 9 gallons of fuel 

into that "half-full" tank, the consumer assumes that there is a total of more than 18-

gallons of fuel in the tank, thus exceeding the fuel tank rating listed in the owner's 

Unusable Volume

Usable Volume

Fuel Line Vapor Head Space

Filler Pipe

Cap 

Passenger Vehicle Fuel Tank Capacity 

Note: The diagram illustrates the parts of a fuel tank that relate 

to tank capacity. The dimensions do not represent the exact scale 

of each tank component. 
manual. The consumer then concludes that the gasoline dispenser must be in error and 

lodges a complaint. 

In addition to considering the factors discussed earlier regarding fuel tank capacity, it is 

important to understand that manufacturers also take into account a number of factors 

when designing the fuel indications. Many variables have an affect on a vehicle's analog 

and/or digital fuel level indications. Auto manufacturers design fuel systems with gauges, 

lights, audible signals, and/or other indications to provide advance warning to the driver 

when fuel levels are low and warrant attention. Motorists would find it inconvenient if 

the fuel gauge needle pointed to "Empty" or the low fuel indicator light was designed to 

illuminate at the very moment the tank runs out of gas, not to mention the possible 

adverse affects to the vehicle's fuel system. In fact, some owner's manuals recommend 

maintaining a minimum level of fuel. 

A shift in fuel levels may occur when the vehicle is not on level ground, for example, 

when parked on a hill or traveling around a curve. A temporary change in the position of 

fuel can cause the fuel gauge needle to shift and/or cause the low fuel warning light to 

flicker or give the appearance of more or less fuel in the tank than is indicated by the fuel 

gauge. Drivers should also keep in mind that the indicated fuel level could vary based on 

the position of the driver's eye when reading the fuel gauge needle on the instrument 

panel. 

A reserve amount of fuel is present in the tank if the manufacturer elects to set the fuel 

gauge to indicate "Full" at a level just below the tank's actual liquid capacity. Similarly, 

reserve fuel may be present in the tank if the manufacturer designs the fuel gauge to 

indicate "Empty" at a level above the actual point where the tank runs out of fuel. 

Some vehicles are also equipped with trip computers to assist the driver in planning travel 

based on the fuel that remains in the tank and mechanisms that estimate remaining 

mileage. These indications are also often used by the driver to determine the "quantity" of 

fuel in the tank prior to purchasing fuel. The vehicle's mileage for each gallon of fuel 

depends on a number of conditions such as vehicle-maintenance, age, cargo/passenger 

load, use of extra accessories such as air conditioning, or the motorist's driving habits, 

terrain, traffic, weather and even fuel composition. Any of these factors can increase fuel 

consumption if the end result is the engine must work harder. Consequently, increased 

fuel consumption will affect how soon the low fuel warning indicator lights up on the 

instrument panel. Trip computers may calculate the estimated miles that can be traveled 

on the fuel remaining in your tank based on the vehicle's fuel economy during a specific 

driving period. The estimated miles that can be traveled may vary since the calculated 

distance might be based on the worse or best case driving conditions that existed when 

the fuel economy was last determined by the vehicle's diagnostic equipment. 

The points outlined in this article may be helpful in addressing complaints in which the 

consumer questions pump accuracy based on the rated capacity of the vehicle's fuel tank 

or the fuel tank indications. However, it is important to recognize that in some such 

instances there may actually be a problem with pump accuracy or its manner of use. 
Consequently, it is prudent to thoroughly investigate all complaints in an attempt to 

determine the cause of the discrepancy. 

If you have any questions about this information, please contact Juana Williams at 301- 

975-3989 or at juana.williams@nist.gov.