Is Petrol or Diesel More Efficient?

For years and years, choosing a new car meant deciding between a fun petrol or an efficient diesel. Nowadays, there are a handful of car fuel alternatives like hybrid tech and fully electric vehicles but petrol and diesel still dominate the market.

One of the most common questions I’m asked is: “Is petrol or diesel more efficient?”

Well, asking whether something is efficient is really rather vague so in this blog, I’ll be comparing petrol and diesel engines across fuel efficiency, performance and running costs. That should give you a good feel for which is the right fuel option for you.

Fuel Efficiency

In the UK, fuel production has focussed on petrol at the expense of diesel. That makes diesel noticeably more expensive compared to other nearby countries. In Germany, for example, diesel is 11% cheaper.

So even though petrol cars achieve lower miles per gallon, it’s not always as simple as it seems since one litre of petrol is cheaper than one litre of diesel.

Here are some average fuel efficiency figures from NimbleFins for both petrol and diesel cars.

  • Petrol: 36 mpg
  • Diesel: 43 mpg

Since diesel is more expensive than petrol, that’s a bit misleading. Yes, a litre of diesel will get you further but it'll also cost you more. At the moment, petrol prices are 125.5 pence per litre and diesel prices are 128.3 pence per litre.

So, here’s how much it’ll cost for you to drive 100 miles in each of those cars if you achieve the fuel efficiency above.

  • 100 miles with petrol: £15.85
  • 100 miles with diesel: £13.56

So diesel is definitely more fuel efficient but the real world cost is slightly less than the MPG would suggest.

Finally, it’s always worth remembering that manufacturers calculate their official fuel efficiency figures in perfect conditions. Out in the real world with headwinds, traffic jams and idiots who break suddenly without warning, you’ll rarely get anywhere near their official figures.

To find out how efficient a car is in the real world, check out What Car?’s True MPG calculator or Honest John’s user-generated Real MPG section.

Performance

Ten years ago, the difference between petrol and diesel engines was clear.

Diesel engines have a much smaller power band so they have a load more torque spread over a shorter rev band. As such, they were good for towing and shifting heavy cars. However, they weren’t much fun and could be a bit loud.

Petrol engines were much smoother and felt a lot more nimble and punchy due to a much taller rev band.

Nowadays, the difference is much smaller. New diesels deliver power a lot quicker and can feel just as sprightly as punchy petrols. When it comes to pure pulling power, however, diesel still reigns supreme. So if you're planning on picking up a 4x4 or SUV, go for the diesel.

Running Costs

All cars have running costs that can be significantly larger than your fuel bill. This is stuff like insurance, servicing, road tax, depreciation and so on.

With leasing, you can ignore a bunch of those costs. The finance company has to deal with the depreciation, the road tax is included, you don’t need an MOT for the first three years

However, you will have to pay for insurance (usually fully comprehensive and GAP insurance) and you will have to pay for servicing and repairs.

Generally speaking diesel cars are more expensive to insure and service, which can eat into (or even wipe out) any savings you make in fuel efficiency.