We use cookies to allow us and selected partners to improve your experience and our advertising. By continuing to browse you consent to our use of cookies. You can understand more and change your cookies preferences here.

When you click on a retailer link on our site, we may earn affiliate commission to help fund our not-for-profit mission.Find out more.

2 August 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 which cars are the most economical and what you need to know about emissions
Adrian Porter
MPG

You can still buy a new petrol and diesel car until 2030, but sales of diesel cars have dropped dramatically and they're slowly vanishing from manufacturer ranges. 

Our latest car survey shows more people are thinking of buying a hybrid for their next car over any other fuel type – is it time to join them?

Should you buy a diesel, petrol or hybrid car?

Today’s hybrid cars use less fuel, produce fewer emissions and are proving more reliable than their diesel and petrol counterparts. 

That’s despite our independent emission tests, which are tougher than the official tests, showing that modern diesel and petrol cars are cleaner than they’ve ever been. But while they may have improved, they’re still being surpassed by hybrid cars in terms of fuel economy and emissions. The exception being motorway fuel economy - this is where diesel still has the edge, but it’s a battle that’s slowly being lost.

Our reliability data comes directly from tens of thousands of UK drivers and reveals that diesel cars are causing their owners the most strife. Petrol cars are more reliable than diesel, but hybrids are the most dependable fuel type of all.

That reliability gap between petrol and diesel only grows as the cars age, with diesels becoming comparatively less and less reliable than petrol and hybrid cars as the years pass. This further reduces the appeal of finding a cheaper (and dirtier) diesel on the used car market.

Are there any reasons left to buy a diesel or petrol car over a hybrid? Keep scrolling to find out as we delve further into the pros and cons of each fuel type, reveal more about fuel economy and emissions, and what owner feedback reveals about the reality of living with different fuel types of car.



Diesel cars vs petrol vs hybrid: in a nutshell

  • Hybrid cars are the most fuel efficient of these 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 evidence of diesel particulate filter (DPF) tampering is found.
  • 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.

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 a weighted average of these scenarios.

  • In our tests, diesel cars trump its petrol counterpart for superior fuel economy in all situations
  • Diesel might cost more per litre at the pump, but currently a diesel car's fuel costs will work out cheaper than the equivalent petrol car
  • However, both diesel and petrol are outdone by full hybrid cars and plug-in petrol-hybrids, which deliver a vastly superior fuel economy around town.

Developments in hybrid technology mean that petrol-hybrids are slowly catching up to diesel on motorway fuel economy.

But our figures show that, on average, people spend around 42% of their driving time around town and 27% on the motorway. Given these averages, the around town and combined mpg will be more important to most people, making hybrids the clear winner (see table below).

As petrol is cheaper than diesel to buy, it's a bit of a double whammy given that the most efficient type of car - petrol-hybrids - uses the least costly fuel. This helps to makes petrol-hybrids substantially cheaper to run than petrol or diesel.

Average fuel economy figures

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, 169 petrol engines, 20 full petrol hybrids. Diesel hybrids are relatively rare and we have not tested enough to present a meaningful average. 

It's important to note these figures do not include plug-in hybrids or mild hybrids due to numbers tested across different car classes, 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)
68.9
72.4
52.3
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

Hybrid fuel economy is at its best when driving around town. But on faster roads such as on motorways, hybrids rely more on their combustion engine which diminishes fuel economy.

However, as hybrid technology evolves, we expect to see more cars with improved overall and motorway fuel economy. As you can see above, some hybrids are already getting there. Which means there will be ever-diminishing reasons to buy a diesel car.

But while hybrids can offer brilliant fuel economy, don't believe the hype around official plug-in hybrid fuel economy claims. In official tests, they're able to offset their fuel economy results with the electric-only range, but to an unrealistic degree.

Our tests also reveal how polluting a plug-in hybrid can be if you're not able to top up the battery yourself. Read more about about our research, hybrids and which models we recommend by heading to our page on the best hybrid cars

Fuel pumps

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.

Are hybrid cars 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 12 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 full hybrids do have are, on average, less severe than the average fault suffered by petrol and diesel cars. Which subsequently means they require less time off the road to be repaired.

We don't yet have enough data to be able to report on plug-in hybrid cars as, like electric cars, they're still establishing themselves. 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 pick of the best hybrid cars.

Petrol and diesel fuel pump

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 than petrol cars in our tests 

For more on the cleanest and dirtiest diesel cars, we reveal all of our emissions results - go to low emission cars.

Are petrol and hybrid cars clean?

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.

Petrol cars also produce more CO2 than diesel. But petrol hybrids produce significantly less climate changing CO2 than both. Looking across all cars we’ve tested, petrol hybrids emit around 24% less than petrol or diesel.

To illustrate the differences between the balance of emissions produced by different fuel types, here is a graphic showing a percentage comparison between the emissions produced by diesel, petrol and full-hybrids we’ve tested in our lab.

Use the arrows or drop-down bar to see how the emission balance differs across example car classes. For the different classes, we’ve also added CO2.

X

(Methodology: based on 329 cars tested on our current test programme. Due to availability, plug-in petrol hybrids used for Mid/Large SUVs and highlighted in a different colour.)

If you want a low emission car and you're not yet ready to make the switch to an electric car, full hybrids and plug-in hybrids (for those who can keep them charged up) are your best bet.

Are electric cars better for the environment?

Electric cars have no engine or exhaust pipe and therefore do not emit any CO2, NOx, CO or PM (some PM will come from tyre wear, as it does with all cars).

Electric cars may not emit CO2, but they aren't squeaky clean. As with any type of car, there is a CO2 cost of generating and delivering the fuel a car needs, including electricity. How much electricity a car needs over a year comes down to its efficiency, and that varies massively from one model to another, and will vary across electric cars.

So you can compare the real CO2 impact of all cars, we work out the CO2 impact of generating and transporting the fuels our cars need, whether it be petrol, diesel, electricity or hydrogen.

We add the CO2 impact of fuel with how efficiently a car uses its fuel in our tests. The result is well-to-wheel (WTW) CO2 figures and it allows us, and you, to compare all fuel types of cars against each other, whether it uses petrol, diesel, electricity or hydrogen.

Find out which cars really are better for the environment and use our free emissions table of WTW CO2 - go to low emission cars

How much car tax will I pay for a diesel, petrol or hybrid in 2021?

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 cars. So diesel and hybrid cars from this era are 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 £155 per year, or £145 for a hybrid car (rates update for April 2021-2022 tax year). 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 registered after this date will no longer cost slightly more to tax in their first year of ownership.

What these car tax changes mean to you

A large portion of the savings offered by more frugal diesel engines were swallowed by the change in car-tax rates from April 2017/2018 onwards.

For example, previously, if you owned a diesel Ford Focus you would pay nothing in car tax for the first year. Then just £30 a year. If your car's CO2 levels were under 100g/km, like some versions of the popular diesel Nissan Qashqai, you would have been exempt from car tax completely. 

Now owners of the same cars have to pay between £160 and £180 in the first year, and then £155 a year afterwards. That's an increase of £125-£155 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.

What is a 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.

Modern petrol cars are also starting to get particulate filters to help them adhere to the latest emission regulations.

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

Cyclist wearing face mask

Will diesel cars be banned?

Yes. Sales of new diesels will be banned along with sales of new petrol cars from 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’.

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

Car emission tests

What is London's 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.