What is a car on-board computer?

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Almost all modern cars have on-board computers, but what are these systems and how do they work?

The first electronic on-board computer appeared in 1978 and was installed on the Cadillac Seville. Mechanical systems had existed before, but the Seville’s ability to calculate its own fuel economy and allow the driver to enter the number of miles to their destination was a novel idea.

Automotive on-board computers have since evolved and become more sophisticated (especially with regard to electric cars), but their principle has remained unchanged. Here we detail what an on-board computer is, how it works and what type of information you can reasonably expect to receive.

What are on-board computers used for?

The trip computer of a Mercedes C63 showing (from left to right, top to bottom) distance travelled, time taken, average fuel consumption and average speed

As the old saying goes, an on-board computer does what it says on the box: it calculates data related to a journey.

An on-board computer provides information on five key areas:

  • Time
  • Distance
  • The rapidity
  • Fuel consumption
  • Interval

Each of these four areas contains layers of information, and while precisely what a trip computer displays will vary from car to car, you can reasonably expect the following:

Time

On-board computers will tell you how long you have traveled since the start of your trip. If they are linked to the satellite navigation system, they can also tell you how far you have to travel before reaching your destination.

Distance

As you can imagine, this will tell you how the car traveled.

The rapidity

This will give you the average speed traveled by the car.

Fuel consumption

This will typically provide two readings: your car’s average fuel mileage and the instantaneous fuel mileage – i.e. how many mpg the car is doing from one moment to the next, allowing you to see how different conditions (e.g. hills, speed, your aggressiveness with the throttle) affect fuel mileage.

Interval

This will tell you how far you can go before your fuel tank runs out. While generally speaking the range figure will decrease as your trip progresses and you deplete the fuel tank, the range figure may actually increase if you start a city trip and then go on the open road where efficiency is best, as the calculations will change depending on your driving as the journey progresses.

On-board computers: Trip A and Trip B

Many trip computers provide information for three “trips”: trip A, trip B, and since reset.

This allows all the information detailed above to be measured separately and in parallel over three periods. You can reset each ride individually or reset all at once.

For example, you can use trip A to determine the car’s distance, speed, and efficiency with each fill-up of fuel, resetting trip A each time you refill the tank.

However, you might also want to see if a new route to work takes less time, offers better fuel economy, or is faster than your usual route. So you can reset trip B to figure that out, while letting trip A continue to run in the background until you need to refuel.

As for the “since reset” information, many people never touch it, which means they can show the distance, speed and efficiency the car has traveled since it left the factory (or perhaps since its battery was last removed).

Also note that if you take a series of short trips (e.g. picking up the kids from school, going to the supermarket, driving to soccer practice), these trips will often be treated as one car trip, which will usually be reset. start a new “trip” after the car has been off for about four hours.

On-board computers for electric cars

Audi e-tron GT screen, showing remaining range

Perhaps the biggest change to come to the world of on-board computers was brought about by the advent of electric cars.

An electric vehicle’s on-board computer will typically show you all of the information covered above, along with the electric equivalent of fuel economy: instead of miles per gallon, you’ll see miles per kilowatt-hour (per example, if you have a 100kWh battery and the car does 3 miles per kWh, you will get 300 miles from a charge – see here for more info.)

On top of that, many EVs will sync their battery ranges with sat nav, calculating how far you can go on a charge and where you’ll need to stop to recharge if you’re going on a long trip.

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