Reading km/h, mph and trip odometers

Two units and three counters sounds like the least interesting part of a speed display, right up to the moment you cross a border, hire a car, or try to work out why your phone says you drove 412 kilometres and the car says 419. Unit conversion is exact arithmetic and holds no surprises. Distance counting is where the interesting differences live, because the phone and the car measure distance in fundamentally different ways and there is no reason for them to agree to the last kilometre.

The conversion, exactly

One mile per hour is exactly 1.609344 kilometres per hour, because the international mile is defined as exactly 1609.344 metres. To go the other way, multiply by 0.621371. The rough mental version most drivers use, add sixty per cent, is good to about one per cent and perfectly adequate for reading a road sign, but a display should always do the exact arithmetic and round once at the end. Rounding twice is how a speed shown in one unit can disagree with the same speed converted from the other.

The speeds worth memorising

The table below covers the limits you actually meet. It is worth reading once before a trip abroad, because the mental arithmetic that works at motorway speed is the least reliable in town, where the numbers are small and the percentage error of a rough conversion is largest.

Why your trip distance and the car's disagree

The car counts wheel rotations and multiplies by a stored circumference, which is a constant the manufacturer chose and which reality only approximates. Tyre wear, pressure, load and temperature all change the true rolling circumference, and the stored value is usually optimistic in the same direction as the speedometer, so a car's odometer tends to over-count by roughly one to two per cent. A satellite trip counter measures distance over the ground, which is closer to true, but it loses a little every time coverage drops out and adds a little noise on winding roads. A disagreement of one or two per cent over a long journey is normal and neither instrument is broken.

How a satellite trip counter accumulates distance

Distance comes from adding up the separations between consecutive fixes, with a filter to reject the tiny jitter that appears when a car is stationary. Without that filter a phone parked overnight would happily invent several hundred metres of driving from noise. With too aggressive a filter, slow crawling traffic is under-counted. Elevation matters too, and a counter that follows the terrain accumulates slightly more distance on a mountain pass than a map-flattened one would. The practical consequence is that a satellite trip figure is accurate to a fraction of a per cent on open roads, and rather less so in a multi-storey car park.

Three counters and what each is for

A total odometer records the whole life of the app and should never be reset; it is the number to look at when someone asks how far this thing has actually measured. Two independent trip counters exist because they answer different questions: one is commonly used per journey and reset every time, while the other runs for a week or a tank of fuel. Keeping them separate is what makes a fuel calculation possible without losing the journey figure. A live trip readout is a fourth thing again, tied to the current drive and starting from zero every time the car moves off.

Average speed is not the average of your speeds

A journey average is total distance divided by total time, and the choice of which time you divide by changes the answer completely. Divide by the whole elapsed period and traffic lights, fuel stops and a coffee are all included, which gives the number that actually predicts arrival times. Divide by moving time only and you get a figure that describes the roads rather than the journey. A display that shows both is being useful; one that shows a single unlabelled average is inviting a misunderstanding. The same distinction explains why a satellite average and the car's trip computer rarely agree: they are usually not excluding the same stopped time, and the thresholds each uses to decide that the vehicle has stopped are different.

Using the numbers for fuel and for expenses

For fuel consumption, brim the tank, reset a counter, drive normally, and brim again: the litres that go in divided by the distance travelled is the true figure, and it will usually be worse than the car's own display, which is calculated from injector timing and inherits the same optimistic distance. For mileage claims, the honest number is the distance actually driven, which is what a satellite counter measures; a per-journey log with a date and a distance is easier to defend than a monthly estimate. The app this guide belongs to keeps the journey history on the phone and can export it, so the record stays yours rather than living on somebody's server.