Car CO2 Emissions Explained: What g/km Costs You

Every car on the UK register carries a single number that quietly decides hundreds of pounds of your motoring costs: its CO2 emissions figure, in grams per kilometre. It's not the same thing as the Euro standard, it isn't about air quality, and it was measured in a laboratory years ago, yet it still sets tax bills today. Here's how the number works and what it costs.

What the number actually is

CO2 emissions correlate directly with fuel burned: roughly, every litre of petrol produces about 2.3 kg of CO2, diesel about 2.6 kg. So the g/km figure is really a fuel-economy measurement wearing different units: a 120 g/km petrol car and a 45 mpg petrol car are close to the same statement.

The figure is certified once, when the model variant is type-approved, and recorded against the car at first registration. It never changes for the life of the vehicle, however the car is driven or maintained. That permanence is what makes it usable for tax, and what makes it slightly fictional as a prediction of your real-world emissions.

CO2 vs the Euro standard: two different numbers

These get mixed up constantly, so one table:

A car can be low-CO2 and zone-banned (an efficient Euro 5 diesel), or zone-compliant and high-CO2 (a Euro 6 V8). Check both before buying. The Euro standards guide covers the other half.

What CO2 costs you in tax

Three different regimes, depending on first registration:

• After 1 April 2017: CO2 sets the first-year rate only: from £10 at 0 g/km to £5,690 above 255 g/km, usually rolled into the on-the-road price. From year two, the flat £200 standard rate takes over. The full table is in our car tax bands guide.
• 1 March 2001 to 31 March 2017: CO2 sets the lettered band (A to M) the car pays every year: from about £20 at the bottom to several hundred pounds at the top. For these cars, a lower-CO2 example of the same model is permanently cheaper to own.
• Before 1 March 2001: no official CO2 figure exists, so VED falls back to engine size (up to or over 1,549 cc), which is why our engine size guide matters for older cars.

CO2 also drives company car tax (benefit-in-kind bands) throughout the car's life, which is why fleet buyers obsess over a few g/km, and why EVs, recorded at zero, dominate salary-sacrifice schemes.

WLTP vs NEDC: why newer figures look worse

Until 2017, CO2 was measured on the NEDC lab cycle: famously gentle, and increasingly gamed. Its replacement, WLTP, uses longer, faster, more realistic test cycles, and the same physical car can certify 15–25% higher under WLTP than NEDC.

The practical consequences:

• Comparing a 2015 car's figure with a 2021 car's figure understates the older car's real appetite. The newer number is more honest, not worse.
• During the transition (roughly 2017–2021), some cars were taxed on converted "NEDC-correlated" figures; the number on the register is the one that counts, whatever a brochure said.
• Plug-in hybrids score spectacularly low official figures (often under 50 g/km) because the test starts with a full battery. Charge it rarely and your real-world figure is nothing like the certificate.

Checking a car's CO2 before you buy

The figure sits on the DVLA record: run the registration through our free car spec check and you'll see the g/km alongside the fuel type, engine size, Euro status and the rest of the official identity. It's also printed on the V5C. For a used purchase, thirty seconds here tells you the first-year or annual tax exposure, worth doing before you fall for the car. Our free car check folds the same figure into the full tax-MOT-mileage picture.

Two honest caveats: EVs show 0 g/km (their tax story changed in 2025; see the car tax bands guide), and any car first registered before March 2001 will show no figure at all, which is normal rather than a data problem.

The bottom line

One lab number, fixed at registration, sets your first-year tax, your annual band on older cars, and company car tax throughout. It's a different axis from the Euro standard that controls zone access, newer WLTP figures read higher because they lie less, and the whole thing is free to look up by registration before you buy.