Your car's built-in fuel economy display lies to you — not maliciously, but structurally. The number it shows is an instantaneous or short-rolling-average consumption figure that responds dramatically to the last few seconds of throttle input. It tells you almost nothing about the actual efficiency of a journey end-to-end. To get a genuinely accurate picture, you need two things together: a GPS record of the trip and a log of real fill-up data.
Why the built-in computer misleads
OBD-calculated fuel economy is estimated from injector pulse width and airflow, which the ECU uses to infer fuel consumed. These estimates are accurate for individual injections but can drift over a long trip. The bigger issue is that the displayed number resets or smooths so aggressively that you can't compare two different routes fairly — the computer forgets the long uphill from this morning by the time you refuel tonight.
Real-world fuel economy is measured simply: fill to the brim, drive until you refuel, fill to the brim again, divide the kilometres driven by the litres added. That denominator doesn't lie. But without a GPS record of the trip, you lose the context that explains the result.
What GPS data adds
Speed and elevation profiles
A GPS speed log shows exactly what speed you maintained on different road sections. Fuel consumption rises steeply above about 110 km/h because aerodynamic drag increases with the square of velocity — so a trip where you averaged 120 on a motorway will always post a worse number than the same distance driven at 100. If you can see the speed trace alongside the consumption figure, you understand the result rather than just observing it.
Trip pattern analysis
Multiple short cold-engine trips use disproportionately more fuel than the same total distance in one run. A GPS trip log makes this visible: if your history shows ten 3 km trips where the engine barely reaches operating temperature, that explains an economy figure that looks poor against the manufacturer's claim.
How to log fill-ups alongside GPS trips
The workflow is simple. After each fill-up, note the odometer reading and the litres added in any notes app or spreadsheet. In PACE, you can add a trip note immediately after recording so the information lives alongside the route. Over time you build a table: date, litres, odometer, and the trip record fills in the rest — distance, average speed, and route type.
Reading the patterns
With even a month of data you can see meaningful patterns: motorway trips at 100 km/h deliver better economy than urban stop-start even if they feel faster, cold-weather trips cost more due to longer warm-up times, and certain routes with long downhill sections flatter the average in a way a flat motorway run doesn't.
Driving style matters more than most people realise
Research consistently shows that anticipatory, smooth driving reduces fuel consumption by 10–25% compared to aggressive acceleration and late braking on the same route. A driving smoothness score from a tracking app correlates surprisingly well with fuel economy — both reward gentle throttle application and early off-throttle anticipation. Improving your score and watching consumption drop at the next fill-up is the most immediately satisfying experiment you can run with this data.
Electric vehicles: the same idea, different metric
For EV drivers, the equivalent metric is kWh per 100 km, and GPS data is even more useful because regenerative braking efficiency varies enormously by route and driving style. A trip over a mountain pass where you regenerate energy on the descent looks very different from a flat motorway run, and seeing both in the context of a speed-coloured route map makes those differences concrete rather than theoretical.