Diesel vs Petrol Car: Which Has a Lower Carbon Footprint?

Overview

When choosing a combustion engine vehicle, the debate usually centers on fuel efficiency versus emissions. For decades, diesel was marketed as the "greener" choice due to its superior fuel economy and lower CO2 output per kilometer. However, the narrative has shifted as we gain a more granular understanding of the total lifecycle emissions and the impact of local air pollutants.

While both petrol and diesel cars rely on fossil fuels, their chemical compositions and combustion processes create distinct environmental profiles. Today, we compare the carbon intensity of driving a standard petrol passenger car versus a diesel equivalent to see which one truly wins the race for the planet.

The Numbers

On a per-kilometer basis, diesel cars generally produce fewer CO2 emissions than petrol cars. According to data from the UK Department for Business, Energy & Industrial Strategy (BEIS), an average medium-sized petrol car emits approximately 192.4g of CO2e per kilometer. In contrast, an average medium-sized diesel car emits roughly 171.8g of CO2e per kilometer.

This represents a roughly 10.7% reduction in greenhouse gas emissions when opting for diesel. However, these figures represent "Well-to-Wheel" emissions, which include both the extraction/refining of the fuel and the tailpipe emissions. While diesel is more efficient in terms of CO2, it is important to note that it often emits higher levels of Nitrogen Oxides (NOx) and Particulate Matter (PM), which, while not primary drivers of global warming, have significant impacts on local air quality and public health.

Why the Difference?

The disparity in carbon footprints between petrol and diesel stems from three primary factors: engine efficiency, fuel energy density, and the refining process.

1. Thermal Efficiency

Diesel engines operate on the compression-ignition principle, which allows for a much higher compression ratio than the spark-ignition systems used in petrol engines. This makes diesel engines inherently more thermally efficient, meaning they convert a larger portion of the fuel's chemical energy into mechanical power rather than waste heat. This is why diesel cars typically achieve 20% to 30% better fuel economy (MPG).

2. Energy Density

Diesel fuel is denser than petrol and contains about 10% more energy by volume. While burning a liter of diesel releases more CO2 (approx. 2.68kg) than burning a liter of petrol (approx. 2.31kg), the diesel engine's superior efficiency more than compensates for this difference, resulting in lower emissions over the same distance.

3. The Refining Trade-off

Both fuels are derived from crude oil, but they require different levels of processing. Refining diesel is slightly less energy-intensive than high-octane petrol, though the introduction of Ultra-Low Sulfur Diesel (ULSD) standards has narrowed this gap. However, the "upstream" emissions—the energy used to get the oil out of the ground and to the pump—remain a significant portion of the total footprint for both.

What You Can Do

If you currently drive a combustion engine vehicle, there are several ways to minimize your footprint regardless of whether you use petrol or diesel:

• Practice Eco-Driving: Smooth acceleration and maintaining steady speeds can improve fuel efficiency by up to 25%.
• Maintenance Matters: Ensure your tires are properly inflated. Low tire pressure increases rolling resistance, forcing the engine to burn more fuel.
• Lighten the Load: Remove unnecessary roof racks or heavy items from the trunk to reduce aerodynamic drag and weight.
• Consider Your Next Move: While diesel has a lower CO2 footprint than petrol, both are significantly higher than Hybrid or Electric Vehicles (EVs). When it’s time to upgrade, looking into electrified options is the single most effective way to slash your transport emissions.

To see exactly how your daily commute or annual road trips contribute to your personal carbon budget, use our tools to get a precise estimate.

Ready to find your personal impact? Calculate your carbon footprint here.