Published emissions and economy figures come from standardised tests, and the gap between those figures and real-world performance prompted substantial regulatory change.
Test cycles
Defined sequences of speeds and accelerations run in laboratory conditions.
Which allows comparison between vehicles tested identically.
Older cycles were criticised as unrepresentative of actual driving patterns.
The gap
Real-world consumption and emissions consistently exceeded laboratory figures.
Which was documented by independent testing organisations over years.
The gap widened over time as manufacturers optimised for the test.
Cycle beating
Designing vehicles to perform well specifically under test conditions.
Which is legal optimisation and was widespread.
Defeat devices detecting test conditions and altering behaviour are a different matter and were the subject of major enforcement action.
Revised procedures
New test cycles designed to be more representative were introduced.
Which produced higher and more realistic published figures.
Comparing figures across the transition requires knowing which procedure produced them.
Real driving emissions testing
On-road measurement using portable equipment, in addition to laboratory tests.
Which addresses the fundamental limitation of laboratory testing.
Conformity factors permitting some divergence from laboratory limits were introduced and have been tightened.
Electric vehicle figures
Range figures come from the same test procedures.
Which means real-world range differs, particularly in cold weather.
Consumption in energy per distance is the comparable figure and is published.
Air quality versus climate
Nitrogen oxides and particulates affect local air quality; carbon dioxide affects climate.
Which are different pollutants with different regulatory regimes.
Policy that reduced one while increasing the other is a documented episode in vehicle regulation.
What buyers should use
Independent real-world testing published by motoring organisations, alongside official figures.
Low emission zones
Restrictions or charges based on vehicle emissions standards.
Which use the standard a vehicle was certified to rather than measured performance.
Evaluations have generally found measurable air quality improvements in affected areas.
Ageing and maintenance
Emissions performance degrades with poor maintenance and component failure.
Which periodic testing regimes are intended to catch.
Tampering with emissions control equipment is unlawful in most jurisdictions and is detectable.
Non-exhaust emissions
Particulates from brakes, tyres and road surface.
Which are unaffected by electrification and are a growing share of vehicle-related particulates.
Regulation of these is at an earlier stage than exhaust emissions.
Lifecycle assessment
Manufacturing emissions matter alongside use-phase emissions.
Which is where comparisons between vehicle types require care.
Published lifecycle studies vary in assumptions and reach broadly consistent conclusions.
What to check
Independent real-world testing figures alongside official ones, and the emissions standard for zone eligibility.
Periodic roadworthiness testing
Annual or biennial testing includes emissions checks in most jurisdictions.
Which catches deteriorated or tampered systems.
Test standards vary and have been tightened for diesel particulates in several countries.
Fleet average requirements
Manufacturers face limits on average emissions across vehicles sold.
Which drives model mix and pricing decisions.
Penalties for exceeding targets are substantial and shape what is offered for sale.
Hybrid figures
Plug-in hybrid official figures assume a proportion of electric running.
Which real-world data has shown is frequently not achieved.
Utility factors used in test procedures have been revised in response to this evidence.
Fuel quality
Fuel specifications affect emissions independently of the vehicle.
Which is regulated separately and varies between markets.
What actually reduces emissions from a given car
Maintenance, correct tyre pressure, driving style and reducing short cold-start journeys.
Why the testing regime changed
Independent measurement demonstrated a systematic and widening gap between laboratory and road performance.
Which was known for years before enforcement action made it politically unavoidable.
The resulting on-road testing requirement addressed the fundamental limitation rather than adjusting the laboratory cycle again.
A final observation
Vehicles were optimised for a test rather than for the road because the test was what was measured and published.
That is a general lesson about measurement rather than a fact about the motor industry, and it recurs wherever a proxy becomes the target.
Second-hand purchases
Emissions standard determines eligibility for low emission zones and affects resale value.
Which is checkable from the registration document.
Vehicles a few years either side of a standard change differ substantially in where they can be driven.
Real-world data sources
Motoring organisations and independent testers publish measured consumption and emissions.
Which is the comparison worth using alongside official figures.
One more thing worth knowing
The independent testing that exposed the gap between laboratory and real-world figures was published for years before it produced regulatory change.
Which is a reminder that the evidence and the response are frequently separated by a long interval and an enforcement action.
The summary
Standardised cycles allowing comparison, historically unrepresentative of road use, now supplemented by on-road measurement.
Which closed most of the gap and required both new procedures and enforcement action to achieve.
Independent real-world figures published by motoring organisations remain the most useful comparison for buyers.