The trip computer: how accurate are its numbers

The fuel consumption figure on the dash looks like a measurement and is a calculation. Understanding what goes into it explains why it disagrees with reality — and by how much.

How consumption is calculated

The control module knows how long the injectors were open and at what pressure. From that it computes the fuel delivered.

It then divides that by distance travelled, taken from the wheel speed sensors.

So there are two quantities, both calculated, and the error in each adds to the total.

Where the error comes from

Wheel size. Distance is derived from rotation and an assumed rolling circumference. Change the tyre size and you change the calculation; covered in the tyre size article.

Tread wear. A worn tyre has a smaller diameter — the car travels less than it counts.

Tyre pressure. Under-inflation changes the effective radius.

Injector tolerance. The calculation uses nominal flow; fouled or worn injectors deliver differently.

Factory optimism. Many manufacturers deliberately calibrate the readout optimistically — a known fact rather than a conspiracy.

The typical result: a trip computer reads lower than reality, usually by a few percent.

How to measure real consumption

There is one method, and it is accurate:

1. Fill to full — to the pump’s first click, on level ground.
2. Reset the trip meter.
3. Use the tank in normal driving.
4. Fill to full at the same pump in the same way.
5. Divide litres by distance and scale to your preferred unit.

What matters: the same station and the same pump, to eliminate differences in the cut-off. One cycle gives a guide, three or four give an accurate figure.

Comparing against the trip computer gives you a permanent correction — after that you can use its readings knowing how optimistic they are.

Range estimates

Computed from tank level and average consumption over a previous period.

The consequence: it lags after a change of regime. Leave the motorway for town and the range still shows motorway figures for tens of miles.

And the reverse: after traffic, joining a motorway makes it pessimistic.

The level sender is covered in the fuel gauge article.

What else the computer shows

Average speed — it includes time stationary with the ignition on, so it reads lower than expected.

Journey time — counted differently on different cars: with or without stops.

Instantaneous consumption — the most useful parameter for learning economical driving, because the response is immediate; covered in the economy article.

What diagnostics adds

The same parameters raw — without the smoothing the cluster applies.

Comparison against fuel trims — if consumption rose, the trims show whether it is a fault; covered in the fuel trim article.

Recording on the move for later review — something the cluster cannot do.

In short

• Dash consumption is calculated from injector open time, not measured.
• The error combines wheel size, tread wear, pressure, and injector condition.
• Factory readouts are usually more optimistic than reality.
• Measure real consumption with two brim fills at the same pump.
• Range estimates lag a change of regime by tens of miles.

Download the app — instantaneous consumption, fuel trims, and fuel level are read with an ELM327 adapter over Bluetooth SPP on Android.

Read next

• Fuel gauge reading wrong: sender, float or instrument cluster
• P0460 to P0463: fuel level sensor circuit faults