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7 January 2021

Petrol vs diesel vs hybrid cars: which is better?

Should I buy a diesel, petrol or hybrid car in 2021? Our expert guide reveals whether diesel really is still the cheaper, wiser choice
Adrian Porter

You can still buy a new petrol and diesel car until 2030, but sales of diesel cars have dropped dramatically and they are slowly vanishing from manufacturer ranges. Our latest car survey shows more people are thinking of buying a hybrid next over any other fuel type – is it time to join them?

It just might be. While our independent emission tests show that modern diesels are cleaner than they’ve ever been, they’re no longer subject to the same tax incentives (unless you buy an old one). Hybrid cars continue to close the gap on fuel economy, and already lead the way when it comes to CO2 (carbon dioxide) emissions.

Based on our extensive data on car ownership, we know that diesel cars are also the most unreliable. Plus our data shows the gap between petrol and diesel reliability only grows as the cars age, further reducing the appeal of finding a cheap diesel on the used car market.

By comparison, hybrid cars are more reliable than anything else, including petrol, diesel and even electric cars.

So is there still a reason to buy a diesel or petrol car today? Here, we tell you whether diesels are really still economical, what you need to know about cars and pollution, and reveal which hybrids give everything else a run for its money when it comes to motorway fuel economy.

Take a look at our expert pick of the best cars to find out which models should be on your shortlist. 

Diesel cars vs petrol vs hybrid: in a nutshell

  • Hybrid cars are now the most fuel efficient of the three fuel types, thanks to being superbly frugal around town and motorway fuel economy continuing to improve.
  • Feedback from tens of thousands of owners in our reliability survey reveals that petrol-hybrid cars are the most reliable (fuel) type of car you can buy.
  • Petrol may not be as reliable as petrol-hybrid, but is substantially more reliable then diesel. The reliability gap increases as the cars age.
  • Diesel cars should no longer cost slightly more to tax in the first year of ownership - as of January 2021 all new cars are Euro 6d compliant.
  • Fewer new diesels are being launched. Some brands have even vanquished big SUV diesel options from their line up.
  • Modern diesel cars have cleaned up their emissions, but many older models remain dirty and high emitters of NOx (oxides of nitrogen).
  • Current MOT rules say your diesel car will fail its MOT if there is visible smoke coming from your exhaust, or they find evidence of diesel particulate filter (DPF) tampering.
  • If you have to replace your DPF, it will be a costly fix.
  • Diesel cars tend to be the fuel of choice for those who need to tow, thanks to diesel engines producing huge amounts of torque (pulling power). But petrol and hybrid alternatives are available.

Still not sure if you should choose petrol, diesel or hybrid? Keep reading to find out.

Which? ignores manufacturers’ fuel-economy claims and publishes more realistic mpg figures from our own tests. Discover the cars we recommend by viewing our round-up of the best cars including best hybrid cars, best small cars and best small SUVs and crossovers.

Diesel vs petrol vs hybrid fuel economy 

Diesel cars have been known for having better fuel economy - and for good reason. In our tests, diesel trumps its petrol counterpart for fuel economy on both the motorway and around town.

Diesel might cost more per litre at the pump, but currently a diesel car's fuel costs will be cheaper than the equivalent petrol car.

However, petrol-hybrids deliver a vastly superior fuel economy around town, which has greatly improved the combined (average) figure. This makes petrol-hybrid cars the most fuel efficient fuel of the three fuel types.

Developments in hybrid technology mean that petrol-hybrids are catching up to diesel on motorway fuel economy. Although our figures show that, on average, people spend around 42% of their driving time around town and 27% on the motorway. Given these figures, the around town and combined mpg will be more important to most people. 

As petrol is cheaper than diesel to buy, it's a bit of a double whammy and makes petrol-hybrids substantially cheaper to run than petrol or diesel.

Average fuel economy figures

Table notes: All results from Which? independent tests, under the latest test programme (2017 onwards). Numbers tested: 135 diesel engines, 168 petrol engines, 21 full petrol hybrids (excludes mild hybrids, plug-in hybrids and all types of diesel hybrids).

Fuel type Average MPG (combined) Average MPG (around town) Average MPG (out of town) Average MPG (motorway)
Petrol-hybrid 53.2 70.8 58 38.7
Diesel 46.8 45.7 53.4 39.8
Petrol 41.9 41.7 48.1 35.5
Table notes: All results from Which? independent tests, under the latest test programme (2017 onwards). Numbers tested: 135 diesel engines, 168 petrol engines, 21 full petrol hybrids (excludes mild hybrids, plug-in hybrids and all types of diesel hybrids).

It's important to note these figures do not include plug-in hybrids or mild hybrids, just 'full hybrids'. Anybody looking to buy a car needs to understand the differences between these types of hybrid and why it's important.

Happily that's exactly what we explain, plus give a rundown of the best hybrids available, with our guide to the best hybrid cars you can buy. 

Which hybrid cars have the best fuel economy?

If you're a Which? member and the table below is locked, please log in

Not yet a member? You can try Which? to unlock our table, below, to discover the hybrid cars with the best motorway fuel economy in our tests. You will also unlock all of our online reviews, including our expert car reviews.

Hybrid cars with excellent motorway fuel economy

Cars Fuel economy (combined) Fuel economy (around town) Fuel economy (motorway)
Log in to reveal 68.9 72.4 52.3
Log in to reveal 67.6 78.5 49.6
Table notes: Results from the current (2017 onwards) test programme. For more on how we measure fuel economy from cars, see how we test mpg and emissions

As hybrid technology evolves, we expect to see more cars with improved overall and motorway fuel economy. Which means there will be ever-diminishing reasons to buy a diesel car.

You can use our independent fuel economy figures from our car reviews, in conjunction with our fuel calculator below, to work out how much a car will cost you to fuel over a year.

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 ‘Tech specs’ part of our independent, expert car reviews.

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

Diesel vs petrol vs hybrid car tax

How much car tax you pay depends on when the car you have was registered to its first owner. The car then stays on that tax system through subsequent re-sales.

Before April 2017: 

All tax rates based on official CO2 emissions. Any car that produces less than 100g/km of CO2 was exempt from car tax. Diesel and hybrid cars, in general, produce low amounts of CO2 compared with petrol. So diesel and hybrid cars were generally cheaper to tax, if not tax free

1 April 2017 onwards: 

Only the first year's payment is based on CO2 emissions. From the second year of ownership onwards, owners pay a standard rate of £150 per year, or £140 for a hybrid car. Only zero emission cars are exempt from car tax.

1 April 2018 onwards: 

Unless diesel cars meet Euro 6d (see how we test mpg and emissions), diesel cars are charged a higher rate in their first year.

1 January 2021 onwards

The emissions standard Euro 6d becomes compulsory for all new cars being sold in the UK. This will mean that diesel cars will no longer cost slightly more to tax in their first year of ownership.

What these car tax changes mean to you

The changes from April 2017/2018 onwards mean a large portion of the savings offered by more frugal diesel engines have now been swallowed by the change in car-tax rates.

For example, under the old system, if you owned a diesel Ford Focus you would pay nothing in car tax for the first year. Then just £20 a year. If your car's CO2 levels were under 100g/km, like certain diesel Nissan Qashqais, you would have been exempt from car tax completely. 

Now owners of the same cars have to pay between £120 and £140 in the first year, and £150 a year afterwards. That's an increase of £120-£140 a year over the older rules.

However, the changes in car tax rates have not been backdated. So if you own a diesel/low CO2 car that was registered to its first owner before April 2017, it will continue to be charged at low CO2.

For current rates and more information on how car tax has changed, see our guide to car tax explained.

Hybrid cars are more reliable than petrol and diesel cars

You might be surprised to find out that a car that has two methods of propulsion would be less problematic than a car with just a combustion engine, but it's true.

Every year, we ask people to complete the annual Which? car survey. We gather details on what faults owners encountered in the twelve months prior to answering our survey.

We find out what faults occur (and reoccur), how severe the faults are, breakdown rates and the time required for cars to be fixed, among other information. In our most recent survey, 47,013 owners told us about 55,833 cars that they own.

The figures are very clear: hybrid owners suffer fewer faults and breakdowns. The faults that hybrids do have are, on average, less severe than the average fault suffered by petrol and diesel cars. Which subsequently means hybrids require less time off the road to be repaired by your friendly neighbourhood mechanic.

Plug-in hybrids, like electric cars, are still establishing themselves. For now. But the more people who buy them and feedback in our subsequent reliability surveys, the clearer the picture we will be able to present of how reliable, or not, these cars are.

Find out more about why hybrid cars are more reliable, how they differ from electric cars and what are the best hybrids you can buy right now by heading to our guide to the best hybrid cars.

Is diesel dirtier than petrol?

Typically, yes. Diesel cars, on average, produce a lot more NOx (Nitrogen Oxide) than petrol cars.

NOx is comprised of NO (Nitric Oxide) and NO2 (Nitrogen Dioxide). NOx is harmful and has been linked to tens of thousands of premature deaths around the world.

It comes out of all cars with a combustion engine (so diesel, petrol and hybrid cars). But looking at figures from our current test programme, despite diesel becoming substantially cleaner in recent years, diesel engines still produce on average:

  • 8.4 times more NOx diesel engines than petrol cars in our tests 

For more on the cleanest and dirtiest diesel cars, see our guide to the low emission cars.

Are petrol and hybrid cars clean?

Not necessarily. They might not produce a large amount of NOx, but with petrol cars comes the potential of high CO (carbon monoxide) emissions.

Just like diesel cars, we penalise petrol cars that produce excessive amounts of CO.  Unless you're ready to switch to an electric car, or even a hydrogen fuel cell cars, petrol-hybrid cars are your best bet.   

To find out more about your car's emissions, use our tool - go to low emission cars

Diesel particulate filter (DPF)

One ongoing issue for diesel cars is that the diesel particulate filter (DPF) can get clogged. When it's in working order, the filter cuts down on harmful particulate emissions from diesel engines being released into the air.

Most owners' handbooks advise running the engine at high speed, for example on a motorway run, to keep the filter clear to avoid it clogging.

Current MOT rules say that any car with a DPF that emits visible smoke, or has found evidence of DPF tampering, will fail its MOT. If your DPF does need replacing, the cost can run into thousands of pounds.

Petrol cars are also starting to get particulate filters, though they are currently nowhere near as common.

Want to know which cars you can trust? We reveal which are the most and least reliable car brands.

Will diesel cars be banned?

Yes. Sales of new diesels will be banned along with sales of new petrol cars in 2030. Sales of new hybrid cars will cease in 2035.

The government’s plan to reduce vehicle emissions requires local councils to use a range of measures such as:

  • Changing road layouts at congestion and air pollution pinch points
  • Encouraging people to buy electric vehicles
  • Retro-fitting buses to give lower emissions
  • Investing in new low-emission buses
  • Encouraging the use of public transport.

If these measures fail, local authorities could introduce restrictions - such as charging zones, or stopping certain cars from using designated roads at set times.

However, the plan also states that these restrictions should be removed in the event that emission levels fall enough to be legally compliant and ‘there is no risk of legal limits being breached again’.

A £255m fund has been set up to help local councils execute their emissions-reduction plans, with £40m being made available immediately.

Several cities around the world have committed to banning diesel cars by 2025.

London Ultra Low Emission Zone and T-Charge

The ULEZ charge affects:

  • Owners of petrol cars that don't meet Euro 4 or better
  • Diesel cars will have to meet Euro 6 (introduced September 2015, mandatory for all new cars as of September 2016, though some cars will have been Euro 6 compliant earlier) to escape this extra charge. 
  • Drivers of older cars that aren't compliant have to pay £27.50 to enter the ULEZ - that's made up of a £12.50 ULEZ charge, plus the £15 congestion zone charge (increased from £11.50 in June 2020).

The ULEZ replaced the T-Charge, introduced in October 2017, and came into force in April 2019. 

The T-Charge meant that any car – petrol, diesel, hybrid or otherwise – that didn't meet Euro 4 emission laws (introduced Jan 2005, mandatory for all new cars as of Jan 2006, though some may have complied earlier than this) had to pay an extra £10 a day on top of the regular congestion charge to drive into central London. 

Wondering whether you should go electric? See our round-up of the best electric cars.

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