What Are CO₂ Emissions — and Why Do They Matter for Drivers?

If you've ever looked at your car details and seen something like “122 g/km” next to CO₂ emissions, you’re not alone in wondering what it means.

That small number actually affects your road tax, your car’s cost, and even whether you can drive in certain areas like London’s ULEZ. Let’s break it down — simply.

Check Your Car CO₂ Emissions in seconds

🧪 What Does “CO₂ Emissions (g/km)” Mean?

CO₂ (carbon dioxide) is the main greenhouse gas released when your car burns petrol or diesel. The more fuel your car burns, the more CO₂ it releases.

So, CO₂ emissions are shown in grams per kilometre (g/km) — meaning how many grams of CO₂ your car produces for every kilometre driven.

A smaller number = a cleaner, more efficient car.

⚙️ How Are Vehicle Emissions Calculated?

Every new car in the UK has its CO₂ emissions tested before it’s sold. These tests take place in a lab on a rolling road, not on a public street. Here’s how it works:

🧾 Step 1: The Test Cycle

Cars are “driven” on a rolling road under different simulated conditions — speeding up, slowing down, idling, and cruising — to copy what real driving looks like.This test is called the Worldwide Harmonised Light Vehicle Test Procedure (WLTP).

Older cars were tested using the NEDC method, which was simpler but gave unrealistically low results.

⛽ Step 2: Measuring the Fuel Burn

While the test runs, sensors track exactly how much fuel is used. The more fuel burned, the more CO₂ is created — because carbon in the fuel combines with oxygen in the air to form carbon dioxide.

📊 Step 3: The Formula

In simple terms:

CO₂ emissions (g/km) = Fuel used per km × CO₂ in that fuel

For example: If a petrol car burns about 0.06 litres of fuel per km, and petrol releases roughly 2,390 grams of CO₂ per litre, then 0.06 × 2,390 = 143 g/km of CO₂.

That’s the figure recorded and sent to the DVLA for your vehicle.

🚗 Step 4: Real‑World Adjustments

Because everyone drives differently — faster, slower, with passengers, in cold or hot weather — the WLTP test also includes corrections to make the result closer to real life. This makes today’s CO₂ ratings more accurate than ever.

🧩 Why It Matters

A car that burns less fuel produces less CO₂. That means lower emissions, cheaper running costs, and often lower tax.

🌫 Why CO₂ Emissions Matter for Drivers

CO₂ emissions affect far more than the environment:

💸 1. Your Road Tax Band

The UK uses your car’s CO₂ rating to decide how much Vehicle Excise Duty (VED) you pay each year. Higher CO₂ = higher tax.

🏙 2. ULEZ & Clean Air Zones

Many cities now charge or ban higher‑polluting cars. Knowing your CO₂ rating helps you check if you’ll be charged.

🚘 3. Resale Value

Low‑emission cars usually sell faster and hold value better. High‑emission vehicles can be more expensive to keep and harder to sell.

🌍 4. Environmental Impact

Cutting emissions helps reduce climate change and air pollution. Every lower‑emission car on the road makes a difference.

🧠 Common Questions

Is 0 g/km always best? Yes — fully electric cars have no tailpipe emissions. But remember, they still have an energy footprint from charging.
Does my CO₂ number ever change? No — it’s fixed when your car is first tested and recorded by the DVLA.
How can I check mine? Use the Resolvo CO₂ Emissions Checker — it’s instant, free and shows your exact emissions, tax band and fuel details.