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Real driving emissions: why lab figures never matched your car
The gap between official figures and reality was not a rounding error. RDE testing exists because the old laboratory cycle could be engineered around.
For decades, emissions and consumption figures came from a laboratory: a vehicle on rollers, following a defined speed profile, at a controlled temperature.
That approach is repeatable, which is genuinely valuable for comparison. It is also predictable — and a test whose exact conditions are known in advance can be optimised for.
Related: fuel consumption: norm or problem and Euro 7 and on-board monitoring.
What the old cycle missed
The laboratory profile involved gentle acceleration, modest speeds, a warm start, no air conditioning, no gradients and no load.
Real driving involves the opposite of most of that. So the gap between official and observed figures was structural rather than dishonest — although it was also, in some documented cases, dishonest.
A car calibrated to be clean under exactly those conditions could behave quite differently outside them, and nothing in the test regime looked.
What RDE does differently
Real Driving Emissions testing puts a portable measurement system on the vehicle and drives it on public roads.
The route must include:
| Segment | Typical requirement |
|---|---|
| Urban | Low speed, frequent stops |
| Rural | Moderate steady speed |
| Motorway | Higher sustained speed |
| Altitude and gradient | Within defined ranges |
| Ambient temperature | Across a specified band |
| Load | Including passengers or ballast |
Emissions are measured throughout, not averaged from a single profile.
The point is that the route cannot be known in advance, so calibrating specifically for it is not possible.
Why cold starts matter so much
One of the most significant inclusions.
A catalyst does almost nothing until it is hot. A particulate filter does not regenerate cold. An SCR system needs roughly 200 °C to convert NOx at all.
The first minutes of a cold start are where a large share of a journey’s emissions occur — and the old cycle used a conditioned start that understated it.
This is also why a vehicle used exclusively for short journeys emits far more per kilometre than the same vehicle used for longer runs. That is not a fault: short trips and engine damage, P2459 DPF regeneration frequency and P20EE SCR NOx catalyst efficiency.
What it means for your own consumption
The most common owner question is why real consumption exceeds the quoted figure. RDE explains part of the answer, and the rest is ordinary physics:
- Cold starts — a warming engine runs rich.
- Short journeys, where a disproportionate share of the distance is cold.
- Speed, because aerodynamic drag rises sharply above about 90 km/h.
- Air conditioning, which is a real load.
- Tyre pressures, and rolling resistance: tyre rotation and wear patterns.
- Load and roof boxes: roof rack and crossbars fitting and loading the car safely.
- Driving style.
Fuel economy myths and eco driving, real fuel savings.
Measuring your own, honestly
The trip computer is a calculation, and it is usually optimistic by a few percent.
The reliable method is brim-to-brim: fill to the first click, note the odometer, drive normally for a full tank, fill again at the same pump, and divide.
Compare that with the trip computer to learn your car’s offset, then use the computer with that correction applied.
Trip computer accuracy and consumption and fuel consumption increased causes.
When rising consumption is a fault
Once you know your genuine baseline, a change from it is diagnostic.
| Symptom | Look at |
|---|---|
| Gradual rise over months | Sensors ageing, fuel trims drifting: fuel trim explained |
| Sudden rise | A specific fault — check for codes |
| Worse only in winter | Normal, partly: winter starting tips |
| Worse with a warning lamp | Read the codes first |
A baseline is what makes any of this meaningful. Without one, “it seems to use more” is untestable.
What RDE does not fix
It measures the vehicle honestly. It does not change the fact that a diesel doing four-kilometre journeys will never regenerate its filter properly, or that a cold engine emits more.
The match between a vehicle and its use remains the owner’s decision, and RDE simply makes the consequences visible.
Diesel versus petrol, which to buy.
The practical next step: pair an ELM327 adapter, open the free tier of the app and log fuel trims and coolant temperature across your typical week. That record shows what your driving actually asks of the car, which is the number RDE was designed to capture.
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