Petrol vs diesel vs hybrid cars: which is better?

If an electric vehicle (EV) isn’t an option for you – for whatever reason, including an inability to charge at home or the high price of many models –  you’ll want to understand more about the alternative options. 

In this guide, we’ll take you through what you need to know about diesels, petrol, full hybrids and plug-in hybrids to help you decide what's right for you.

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Should you buy a diesel, petrol, full hybrid or PHEV?

We’ll go into more detail below, but if you want to know what sort of car you should buy, here's a quick summary:

• Full hybrids are fuel efficient and the most reliable type of car you can buy.
• Plug-in hybrids (PHEVs) can be extremely cheap to run if you can charge them at home and make short journeys.
• Petrols and diesels aren’t necessarily uneconomical, but it depends which car you choose.
• Mild hybrids are worth a look, but they don’t save as much fuel as you might think.

Before we get started on which is best, some definitions...

Mild hybrids

Mild hybrids use a very small battery and electric motor to recover energy from deceleration and braking, but the diesel or petrol engine is always on. In our data we put these in the same category as pure petrol and diesel cars since the effect of a mild hybrid on performance and economy is subtle. 

Full hybrids

Full hybrids have larger batteries and motors that can power the car in an electric-only mode at lower speeds (usually around 30mph) for a mile or so.

Plug-in hybrids (PHEVs)

Plug-in hybrids are more akin to EVs as the batteries and motors can power the car at high speeds for long distances (usually between 20 and 80 miles). However, to get the best out of them you need to keep them fully charged.

We reveal the best hybrid cars and explain the difference between a full hybrid, plug-in hybrid and mild-hybrid.

Do diesel, petrol or hybrid cars have the best fuel economy? 

Diesel cars have been known for having better fuel economy – and our test figures support that. In our lab, we look at how economical cars are in a range of scenarios: driving around town, out of town and on the motorway. We also present a 'combined' figure, which is based on a single 62-mile test on a mix of simulated road types. 

Our annual car survey shows that, on average, people spend around 46% of their driving time around town, 28% on out-of-town roads and 26% on the motorway. Given these averages, the around town and combined mpg will be more important to most people. While full hybrids come out on top here, keep in mind that if you are able to keep a PHEV fully charged at home and only make short journeys, you’ll rarely have to buy petrol or diesel, making running costs as cheap as a full battery EV.

Some PHEVs can only do 20 miles (or less) on a single charge, while top-spec models can do over 60 miles. As such, it’s important to match the PHEV you buy with your lifestyle. And if you can’t charge at home, you may find yourself spending more on charging your PHEV than you would if you’d bought a normal petrol or diesel car. If you don’t keep it charged, you’ll be lugging around a battery without benefitting from its presence at all, making it the least efficient way of getting around. 

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Average fuel economy figures for petrol, diesel and hybrid cars

Table notes: All results from Which? independent tests, under the latest test programme (2017 onwards). Numbers tested: 169 diesel engines, 229 petrol engines, 55 full petrol-hybrids, 23 mild diesel-hybrids, 50 mild petrol-hybrids, 58 petrol PHEVs. * A combined 62-mile cycle. For PHEVs, the car is fully charged at the start of the cycle. **Town, out of town and motorway mpg figures based on PHEVs with an empty battery. Data correct as of 12 February 2025.

Fuel cost calculator: how much will you pay?

You can use our simple fuel cost calculator, below, to easily work out the fuel costs between two cars. Enter the respective fuel-economy figures for the cars and your mileage to see the difference. 

For the most accurate results, don't use the manufacturers' claimed figures. Instead, use our realistic mpg data from the ‘specifications’ part of our independent, expert car reviews.

Current fuel prices can be looked up on sites such as TheAA.com or PetrolPrices.com.

Are hybrid cars more reliable than petrol and diesel cars?

The latest results of our annual car survey, which covered more than 70,000 individual cars, show that full hybrids suffered fewer faults in the year covered by our survey than any other type of car. 

This is true both for cars aged 0-4 years and 5-9 years. 

Plug-in hybrids, meanwhile, are among the least reliable type of car, alongside diesels, suffering from more faults and more total breakdowns than all the types of engine. Older PHEVs are particularly problematic; 12% experienced a breakdown in the 12 months covered by our survey, which is well above the average of 7% for all fuel types. PHEVs also spend longer off the road on average when they are faulty; 5.3 days per year for newer PHEVs, and 4.6 for older PHEVs (compared to overall averages of 4.8 and 4.0 respectively). 

While this doesn’t make for great reading for PHEV fans, keep in mind that these are averages. We found individual PHEVs that did well in our survey, showing you should pick carefully. You can see individual reliability ratings for over 300 cars in our most reliable cars guide, or alternatively read about the most reliable car brands (including those who make PHEVs). 

Is diesel dirtier than petrol?

Diesel cars, on average, produce a lot more NOx (nitrogen oxide) than petrol cars.

NOx comprises NO (nitric oxide) and NO2 (nitrogen dioxide). Both of these are harmful to human health and road transport is the biggest generator of nitrogen oxides in the UK, ahead of power stations.

NOx comes out of all cars with a combustion engine (so diesel, petrol and hybrid cars). However, looking at figures from our current test programme, and despite diesel becoming substantially cleaner in recent years (thanks to the mandatory use of the exhaust fluid AdBlue in all new diesels since 2015), diesel engines still produce substantially more NOx than cars with petrol engines. 

Conversely, petrol engines – be it pure petrol or full hybrids – produce a lot more carbon monoxide than diesels, but this is still a very small amount overall. 

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