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A trip computer is an onboard computer device fitted to cars which can generally record distance travelled, average speed, average fuel consumption, and display real time fuel consumption information. This computer can be found in various vehicles.
The first mechanical trip computers such as the "Halda Speedpilot (2004)", produced by a Swedish taximeter manufacturer, were made in the 1950s as a car accessory to enable the driver to maintain a given time schedule, particularly useful in rallying. One was installed as standard in the 1958 Saab GT750. In 1978, General Motors Cadillac division introduced the "Cadillac Trip Computer" available on the Cadillac Seville. Electronic ones were fitted in General Motors products, and are still usually reserved for more upscale cars, although some lower-end models are fitted with them often as an option. They can range from basic to complex. The most basic varieties of the trip computer incorporate average fuel mileage and perhaps an outside temperature display. Middle-of-the-road versions will often incorporate trip information into a bundle and include information on fuel, speed, distance, cardinal heading (compass), and elapsed time. The most advanced trip computers are reserved for high-end cars and often feature average calculations for two drivers, a stop watch, tire pressure information, an over-speed warning tone, as well as a multitude of other features.
Sometimes the trip computer display will be incorporated into the gauge cluster, into the dashboard or navigation system screen, or in an overhead console. Some vehicles will convey maintenance information to the driver to inform them of scheduled maintenance. The current Acura TL does this in stages, first alerting the driver with a 'Due Soon' message. Once the programmed mileage is reached, it alerts 'Due Now'. If the vehicle is driven past a certain point, the message will change to 'Past Due'. Mercedes-Benz vehicles constantly monitor the quality of the oil and alert the driver of the need of a change when the oil degrades to a certain extent.
Some trip computers can display the diagnostic codes mechanics use. This is specially useful when the mechanic wants to see what the codes are while driving the car. In 2004 Linear Logic developed the ScanGauge which at the time was the only easily installed (via OBDII) accessory that works as a trip computer, 4 simultaneous digital gauges, and a diagnostic trouble code reader. This device has available 12 different measurements which can be used as the 4 digital gauges. The units of measure can be independently selected between miles/km, gallons/liters, Celsius/Fahrenheit, and PSI/KPA.
See also 
- "Arduino Forum - OBDuino, onboard diagnostic for your car". Arduino.cc. Retrieved 2009-12-08.