Fuel Gauge - Wikipedia

Instrument showing volume in a fuel tank

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In automotive and aerospace engineering, a fuel gauge is an instrument used to indicate the amount of fuel in a fuel tank.[1] In electrical engineering, the term is used for integrated circuits (ICs) determining the state of charge of batteries.

Motor vehicles

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The gauge consists of two parts:

• The sending unit – in the tank
• The indicator – on the dashboard

The sending unit usually uses a float connected to a potentiometer, typically printed ink design in a modern automobile. As the tank empties, the float drops and slides a moving contact along the resistor, increasing its resistance.[2] In addition, when the resistance is at a certain point, it will also turn on a "low fuel" light on some vehicles.[3]

Meanwhile, the indicator unit (usually mounted on the dashboard) is measuring and displaying the amount of electric current flowing through the sending unit. When the tank level is high and maximum current is flowing, the needle points to "F" indicating a full tank. When the tank is empty and the least current is flowing, the needle points to "E" indicating an empty tank; some vehicles use the indicators "1" (for full) and "0" (for empty) or "R" (for reserve) instead.[4]

The system can be fail-safe. If an electrical fault opens, the electrical circuit causes the indicator to show the tank as being empty (theoretically provoking the driver to refill the tank) rather than full (which would allow the driver to run out of fuel with no prior notification). Corrosion or wear of the potentiometer will provide erroneous readings of fuel level. However, this system has a potential risk associated with it. An electric current is sent through the variable resistor to which a float is connected, so that the value of resistance depends on the fuel level. In most automotive fuel gauges such resistors are on the inward side of the gauge, i.e., inside the fuel tank. Sending current through such a resistor has a fire hazard and an explosion risk associated with it. These resistance sensors are also showing an increased failure rate with the incremental additions of alcohol in automotive gasoline fuel. Alcohol increases the corrosion rate at the potentiometer, as it is capable of carrying current like water. Potentiometer applications for alcohol fuel use a pulse-and-hold methodology, with a periodic signal being sent to determine fuel level decreasing the corrosion potential. Therefore, demand for another safer, non-contact method for fuel level is desired.

Moylan arrow

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Since the early 1990s, many fuel gauges have included an icon with a fuel pump and an arrow, indicating the side of the vehicle on which the fuel filler is located.[5][6] The use of the icon and arrow was invented in 1986 by Jim Moylan (1944–2025),[7] a designer for Ford Motor Company.[8] After he proposed the idea in April 1986,[9] the 1989 Ford Escort and Mercury Tracer were the first vehicles to see it implemented. Other automotive companies noticed the addition and began to incorporate it into their own fuel gauges.[5][6]

Aircraft

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Magnetoresistance type fuel level sensors are becoming common in small aircraft, and offer a potential alternative for automotive use. These fuel level sensors work similar to the potentiometer type, but a sealed detector at the float pivot determines the angular position of a magnet pair at the pivot end of the float arm. These are highly accurate, and the electronics are completely outside the fuel. The non-contact nature of these sensors address the fire and explosion hazard, and also any issues related to fuel combinations or additives to gasoline such as alcohol fuel mixtures. Magneto resistive sensors are suitable for all fuel or fluid combinations, including LPG and LNG. The fuel level output for these senders can be ratiometric voltage or preferable CAN bus digital. These sensors are also fail-safe in that they either provide a level output or nothing.

Systems that measure large fuel tanks (including underground storage tanks) may use the same electro-mechanical principle or may make use of a pressure sensor,[10] sometimes connected to a mercury manometer, or an ultrasonic probe.

Many large transport aircraft use a different fuel gauge design. An aircraft may use a number (around 30 on an A320) of low voltage tubular capacitor probes where the fuel becomes the dielectric. At different fuel levels, different values of capacitance are measured and therefore the level of fuel can be determined. In early designs, the profiles and values of individual probes were chosen to compensate for fuel tank shape and aircraft pitch and roll attitudes. In more modern aircraft, the probes tend to be linear (capacitance proportional to fuel height) and the fuel computer works out how much fuel there is (slightly different on different manufacturers). This has the advantage that a faulty probe may be identified and eliminated from the fuel calculations. In total this system can be more than 99% accurate. Since most commercial aircraft only take on board fuel necessary for the intended flight (with appropriate safety margins), the system allows the fuel load to be preselected, causing the fuel delivery to be shut off when the intended load has been taken on board.

Usage in fleet and fuel management

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The data from a vehicle's fuel gauge is used as telemetry in telematics for modern fleet management and fuel-management systems. A factory-installed telematic control unit or an aftermarket GPS tracking unit collects the fuel level data, alongside other information, and transmits it to the central software of a vehicle tracking system. This integration is an aspect of fleet digitalization, the evolution from simple tracking to business intelligence.[11] This allows fleet managers to:

• Track fuel consumption and efficiency across the entire fleet.
• Monitor for instances of potential gasoline theft.
• Plan refueling stops as part of a journey planner.

However, the standard OEM fuel gauges in many vehicles are not designed for high-precision measurement. They are often built to provide a general indication to the driver and can be inaccurate, especially on uneven terrain or due to the irregular shape of the fuel tank.[12]

Because of this lack of precision, many commercial fleet operators install high-precision aftermarket fuel level sensors (e.g., capacitive or ultrasonic sensors) directly into the fuel tank. These specialized sensors provide more accurate data, allowing the fleet telematics system to precisely track every liter of fuel added during refueling and to detect the sudden drops in fuel level that indicate siphoning or theft.[13]

Fuel gauge integrated circuits

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In electronics there are different integrated circuits available,[14][15][16] which control the current state of charge of accumulators. These devices are also called "fuel gauge".

See also

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• Ammeter
• Float switch
• List of auto parts
• List of vehicle instruments
• Sight glass

References

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1. ^ Erjavec, Jack (2005). Automotive Technology. ISBN 1-4018-4831-1.
2. ^ "How Fuel Gauges Work". 4 April 2001.
3. ^ William Harry Crouse; Donald L. Anglin (March 1981). Automotive fuel, lubricating, and cooling systems. Gregg Division, McGraw-Hill. pp. 35–36, 155. ISBN 978-0-07-014862-8.
4. ^ James E. Duffy (1987). Auto Fuel Systems. Goodheart-Willcox Company. pp. 126–128. ISBN 978-0-87006-623-8.
5. ^ a b "Do You Need to Warm Up Your Car? Plus, a Teeny, Glorious Car Hack", Every Little Thing Podcast, 8 October 2018
6. ^ a b Jason Torchinsky, "The Inventor of the Little Arrow that Tells You What Side the Fuel Filler Is On Has Finally Been Found", Jalopnik.com, 8 October 2018
7. ^ "James N. Moylan obituary", including video of his memorial service, December 28, 2025, Vermeulen-Sajewski Funeral Homes, Michigan
8. ^ Cohen, Ben (3 January 2026). "The Genius Whose Simple Invention Saved Us From Shame at the Gas Station". The Wall Street Journal. Retrieved 3 January 2026.
9. ^ Farley, Jim. "June 4, 2002 tweet". Twitter. Retrieved 22 February 2023.
10. ^ The Aeronautical Journal. Royal Aeronautical Society. 1976. p. 331.
11. ^ "From Tracking To Intelligence: The Next Leap In Fleet Digitalization". Forbes. 20 June 2025. Retrieved 29 September 2025.
12. ^ "Fuel consumption monitoring through telematics: an investment that pays off". AMS-SAM. Retrieved 29 September 2025.
13. ^ "Fuel Management Helps to Optimize Municipal Fleet Operations in Belgrade". Omnicomm World. Retrieved 29 September 2025.
14. ^ "Battery fuel gauges". TI.com.
15. ^ "Battery Fuel Gauge – Gas Gauge". MaximIntegrated.com.
16. ^ "Battery Monitoring – Fuel gauge ICs". STMicroelectronics.

External links

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• Media related to Automobile fuel gauges at Wikimedia Commons
• "The Fuel Gauge and Tank Unit", explanation of operation of double coil moving iron indicators

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