Most economical cars: best mpg cars of 2021

The best mpg cars with the lowest CO2 emissions: these are the most economical cars - with petrol or diesel engines - available now

BuyaCar team
Mar 25, 2021

Looking for a car that's going to save you the most money long-term? You'll want to be searching for the most economical cars on the market. One of the more substantial aspects of a car's running costs is fuel economy - which is often measured in miles per gallon (mpg). Obviously if you wanted to drive the most economical cars then you'd grab the keys to an electric car. But if you want to be economical and have the reassurance of a conventional petrol or diesel engine, then the most economical cars available to you are plug-in hybrids.

A plug-in hybrid car supplements a combustion engine with a large battery that can be charged up for generally around 20-30 miles of zero-emission electric motoring. When battery power runs low, however, a petrol or diesel engine takes over so you don't have to stop and recharge. When battery usage is maximised fuel economy can be pushed upwards of 150mpg, while carbon dioxide (CO2) emissions can drop below 50g/km, which means these cars sit in very lowest company car tax bands. In London, they are all exempt from the congestion charge.

If you can keep the car charged regularly, then the fuel savings should roll in, but plug-in hybrid won't be for everyone. They're comparatively expensive to buy and much less economical over longer distances, once the batteries are empty the extra weight of the electrical elements means fuel economy will be worse than a standard petrol or diesel model.

Read on for our pick of the most economical cars, based on official mpg figures. We've used figures from the latest test method, known as WLTP, which is more accurate than earlier procedures. They still don't fully reflect real world driving, though, and that's particularly the case when it comes to plug-in hybrids. Wheel sizes and equipment levels can also affect fuel economy - so actual figures can vary pretty much from model to model.

Most economical cars 2021

1. Hyundai Ioniq

Best economical car for efficiency on all journeys

Our pick Hyundai Ioniq Hybrid SE
Used deals from £12,000
Monthly finance from £212*
Fuel economy
CO2 emissions 26g/km

The large battery packs in plug-in hybrid cars usually makes them several thousands of pounds more expensive than standard models but you can pick up a nearly-new Ioniq PHEV from  and a new model for less than £26,000, which brings it into line with some diesel cars. It's certainly more efficient than them, even if the official fuel economy figure doesn't really reflect what you can expect.

When fully charged, the Ioniq has a range of 25 to 30 miles on electric power alone, although the petrol engine does power up to provide a boost when you push the accelerator hard. Beyond battery power, the car remains frugal, with motorway fuel economy of 70mpg possible if you drive smoothly. That's not difficult as the engine lacks the power for speedy acceleration. Hyundai also makes a non-plug-in version of the Ioniq, which is less efficient but cheaper, with prices starting from £212 or £212 per month.

We've gone for the standard SE model here, as it offers you plenty of tech and keeps the price down somewhat.


2. Toyota Prius

Best economical car for futuristic design

Our pick Toyota Prius 1.8 VVT-i Active with 15-inch wheels
Used deals from £15,750
Monthly finance from £228*
Fuel economy
CO2 emissions 28g/km

The Prius' official mpg figure is a stratospheric 235mpg but you can even improve on that if your commute is less than 30 miles long, because the car will cover virtually all of that distance on electric power alone. Even when the petrol engine starts up, fuel economy is still pretty good - gentle driving can return 70mpg - much like the Ioniq - on a long motorway drive. The price for this is fairly sedate performance.

If you live and mainly drive in the city, then the chances are that you'll rarely need to visit a petrol station, but the Prius' all-round efficiency should result in low-cost motoring, however it's used. The minimal CO2 figure is outstanding but given that the lowest band of company car tax is 1-50g/km CO2, it won't earn you any additional tax saving to many other cars on this list.

Again, we've gone for the standard trim level here to keep a lid on the cost. There's still plenty of equipment in Active models though and the 15-inch wheels will boost fuel economy even more.


3. Kia Niro

Best economical car for a high driving position

Our pick Kia Niro Plug-in Hybrid '2'
Used deals from £14,990
Monthly finance from £241*
Fuel economy
CO2 emissions 31g/km

With hybrid power and a high driving position, the Kia Niro Plug-in Hybrid should be the perfect car for modern roads, offering excellent fuel economy and a crossover design, which should offer the visibility and practical benefits of an off-road car, with the comfort and stability of a standard hatchback. It’s one of the more affordable plug-in hybrid cars too, with new prices starting at just over £30,000 and used models available for less.

The hybrid system will power the car on electric energy along for around 25 miles. It does reduce boot space to 324 litres - less than you’ll find in a VW Polo, but the passenger area is spacious enough to carry adults in the front and back. For more boot space, but reduced fuel economy, a non-plug-in hybrid Niro is available, costing from £14,990 or £241 per month. Like all Kias, the Niro has a seven-year warranty - limited to the first 100,000 miles - providing plenty of reassurance that it will last.

The entry-level '2' trim should once again prove ample for most, with more than enough tech to keep you busy during even the longest of journeys.


4. Mercedes E-Class

Best economical car for diesel and petrol options

Our pick Mercedes-Benz E300de AMG Line
Used deals from £43,879
Monthly finance from £580*
Fuel economy
CO2 emissions 41g/km

If you're looking for an efficient Mercedes E-Class, then you have the choice of a petrol or diesel plug-in hybrid car. Both the Mercedes E 300de (diesel) and E 300e (petrol) have official fuel economy figures of more than 150mpg and carbon dioxide emissions that put them into the lowest company car tax bracket, although the diesel version is subject to a 4% surcharge.

This makes the petrol car the best choice for anyone looking for the lowest company car tax while the diesel version is the pick for buyers who are looking for the best fuel economy in all situations: an electric range of around 30 miles in real-world conditions (take off around two miles if you choose the estate car) is ideal for short commutes, while the efficiency of the diesel engine will cut costs on longer journeys.


5. Kia Optima Sportswagon

Best economical car for efficiency with a long warranty

Our pick Kia Optima Sportswagon PHEV 2.0 GDi
Used deals Limited stock
Fuel economy
CO2 emissions 34g/km

The Kia Optima doesn’t just have a range of around 30 miles on electric power alone, but an efficient petrol engine too, so fuel economy remains good even when the battery is low and the engine fires up. You do feel that the car has been engineered for efficiency - unfortunately in its slow acceleration. The most efficient version is the Sportswagon estate, but there's also an Optima saloon with a smaller boot and reduced electric range.

The Optima PHEV benefits from Kia’s seven-year / 100,000 mile warranty but not the manufacturer’s traditionally cheap prices from new. However, used prices have dropped to below £20,000, which is equivalent to a Mitsubishi Outlander PHEV of a similar age and only slightly cheaper than a BMW 330e. Both cars are subject to the same rate of company car tax.


6. Mitsubishi Outlander PHEV

Best economical car for SUV practicality without the price premium

Our pick Mitsubishi Outlander PHEV 4h
Used deals from £13,000
Monthly finance from £257*
Fuel economy
CO2 emissions 40g/km

This five-seat, four-wheel drive off-roader has been exceptionally popular with buyers looking to pay the lowest level of company car tax because of low CO2 emissions and a grant from the government that reduced the price. It’s good for economical short journeys, with the ability to drive for up to 32 miles in all-electric mode. But it’s much less efficient on long journeys, when the petrol engine has to work hard to keep the heavy SUV moving.

Recent updates have kept the design of the Outlander looking fresh, but the car is being caught up by more modern competitors, such as the Kia Niro PHEV, above, with a more comfortable ride and better-quality interiors. The price advantage is also reducing too, now that the government has removed the grant for plug-in hybrid vehicles. All-electric models, such as Kia's eNiro and the Hyundai Kona Electric have a similar price, and will also benefit from much lower company car tax next year.

You could quite happily get away with entry-level Juro models as they do come with plenty of in-car tech, but we think the additional safety tech offered with the slightly pricier 4h trim give the Outlander PHEV a more rounded feel.


7. BMW 5 Series

Best economical car for dapper looks and driving fun

Our pick BMW 530e M Sport
Used deals from £20,299
Monthly finance from £306*
Fuel economy
CO2 emissions 49g/km

BMW's plug-in hybrid 5 Series just squeezes in to the lowest company car tax bracket (until next year), bringing improved fuel economy without dramatically affecting the sporty driving experience that makes the car popular with buyers.

A fully-charged battery will take the car around 20 miles in normal driving, with barely any assistance from the petrol engine, but the car's automatic mode is best for fuel economy on longer trips, as it switches between engine and motor for the most efficient performance.

Performance is the right word, as the car accelerates from 0-62mph in a swift 6.2 seconds and retains its agility when changing direction, which makes it fun to drive. You will have to put up with a slightly bumpier ride on rough surfaces - due to the extra battery weight - and a smaller boot, which offers roughly as much space as a Volkswagen Golf.


8. BMW i8

Best economical car for speed

Used deals Limited stock
Fuel economy
CO2 emissions 42g/km

The BMW i8 first appeared five years ago, but it still turns heads, as the embodiment of BMW's belief that electric and hybrid cars can remain exciting to drive. Of course, if you do drive it in the enthusiastic manner intended, then fuel economy is not going to be particularly impressive: the electric motor and petrol engine are best-suited to working together, delivering instant acceleration that continues to the legal limit and beyond.

Even so, switch it to eDrive mode and you can drive the i8 for around 30 miles on electric power alone. This makes it more efficient than a typical supercar. Restrained use of the accelerator on longer trips should result in fuel economy that matches a Ford Focus, but don't expect the 100mpg-plus figures that the official tests record.


9. Mercedes S-Class

Best economical car for S-Class enthusiasts

Our pick Mercedes-Benz S-CLass 560e EQ Power Premium Plus
Used deals Limited stock
Fuel economy
CO2 emissions 57g/km

There's no surprise that the company car tax advantages of this plug-in version of the Mercedes S-Class are what makes it particularly attractive: Additional Rate 45% taxpayers will save almost £4,500 per year compared with a standard diesel model.

But the luxury car is also likely to do its bit to reduce urban emissions, as it transports executives from one city meeting to another in sumptuous surroundings: the electric range of around 30 miles with a full battery may be enough for a whole day's schedule. A home charger will take around three hours for a full recharge.

Efficiency drops off considerably once the battery is exhausted and the large petrol engine takes over; the diesel model is better for long motorway journeys. The car does its best, though. When you - or more likely your driver - are behind the wheel,. you'll see prompts to lift off the accelerator at just the right moment, to allow you to coast to a junction or a slower speed limit, avoiding the need to brake hard and waste energy.

If you're going to be spending big money on a hybrid E-CLass, you might as well do it properly and skip the cheapest trim levels. Premium Plus is a good place to start, with a panoramic sunroof, an automated boot lid, an upgraded sound system and a seat massage function.


10. Volvo V90

Best economical car for Scandi-chic

Our pick Volvo V90 T8 Plug-in Hybrid Inscription
Used deals Limited stock
Fuel economy
CO2 emissions 49g/km

Volvo estate cars are no longer the biggest in the business. For a brand that made its name with cavernous square-shaped load carriers, that could have come as an embarrassment if it hadn't focused on standing out in a totally different area: style.

For while the BMW 5 Series Touring, Mercedes E-Class Estate and Skoda Superb Estate all offer more luggage space, it's not particularly controversial to suggest that the V90 is much better looking. That goes for the interior too, where a large touchscreen controls virtually every function, leaving calm minimalism across the rest of the dashboard. It no longer looks as cutting edge as it did when the car was first launched, but still creates a calming effect as soon as you open the door.

You can expect around 20 miles of range on electric power alone from the hybrid T8 model - 10 miles short of the official figure. Longer distances will see fuel economy tumble, as the petrol engine hauls the two-ton car on its own. Given the car's price of almost £60,000 when new, before discounts, and the limited availability of used models, it's only likely to appeal to company car users who will feel the benefit of the 49g/km CO2 emissions that bring the lowest rate of company car tax. And despite being overtaken by some rivals, the V90's boot is still pretty enormous.


*Representative PCP finance - Ford Fiesta:

48 monthly payments of £192
Deposit: £0
Mileage limit: 8,000 per year
Optional final payment to buy car: £2,923
Total amount payable to buy car: £11,926
Total cost of credit: £2,426
Amount borrowed: £9,500
APR: 9.9%

BuyaCar is a credit broker, not a lender. Our rates start from 6.9% APR. The rate you are offered will depend on your individual circumstances.


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