Fuel is the second biggest car ownership cost youll face after depreciation. On average, 25% of running costs involve filling up, but this can be as high as 50%, depending on the engine.
Its little wonder then that fuel economy is the primary concern for What Car? readers ahead even of purchase price and insurance premiums. Yet because engine efficiency rarely lives up to the Government mpg figures you see in the brochure, comparing cars can be nigh-on impossible.
Thats why we launched True MPG in spring 2012, to give you a realistic idea of what sort of fuel bills you can expect. Unlike the laboratory-derived Government figures, True MPG data is gained through testing in real-world conditions.
One year on from its launch, True MPG is now a vital part of the buying process. Our data shows that basing your decision on True MPG, rather than official figures, could save you thousands at the pump.
The difference between the most and least fuel-efficient engine weve tested is 49.2mpg, or around 70%. However, this takes into account everything from ultra-powerful supercars to super-frugal city cars.
Click here to see the full version of our What Car? True MPG infographicWeve found that cars will use an average of 17.3% more fuel than Government figures suggest, which at average mileage rates of 12,000 miles a year over three years equates to around 660 more than you'd have expected to pay for fuel.
Of course, averages mask a vast variance in stated and actual economy. Buy a Mazda 3 MPS, for example, and youll see it has a True MPG that beats its Government figure meaning that over three years your petrol bill could be around 493 cheaper than expected.
At the other end of the scale, a 1.5-litre Renault Megane with stop-start could cost you 1663 more in fuel than Government figures would have you believe.
Does car size matter?
Its worth remembering that just because a car doesnt perform as well as its official figures suggest it should, that doesnt mean its uneconomical. In fact, the most fuel-efficient car weve tested the Kia Rio 1.1 CRDi 1 Ecodynamics has a True MPG thats 20% lower than its Government figure. It delivers a staggeringly impressive average economy of 70.6mpg in real-world driving conditions.
Looking at car segments alone, our data shows that city cars which on average use around 23% more fuel on the road than the Government figures state are the most economical, with an average True MPG of 48.6. This is compared with SUVs, for example, which on average will deliver around 40.7mpg.
Superminis, meanwhile, have an average True MPG of 44.2 compared with a Government average of 58.6mpg for the sector.
As weve already shown, though, these averages mask a huge variance in actual performance. The best-performing supermini in our test was the 1.1-litre diesel Kia Rio with its True MPG of 70.6 some 60% more efficient than the average car in its class.
In fact, its the small cars that have proved most fuel efficient in our emissions testing, with the top five True MPG performers including the Kia Rio, Ford Fiesta and Fiat Panda.
Top five True MPG performers
1. Kia Rio 1.1 CRDi 1 Ecodynamics 5dr 70.6mpg
2. Citroen C3 Picasso 1.4 e-HDi 70 Airdream VTR+ 70.3mpg
3. Ford Fiesta 1.6 TDCi 95 Econetic (old model) 62.1mpg
4. Fiat Panda 1.3 Multijet 75 61.0mpg
5. Mercedes-Benz A200 CDI Blue Efficiency 59.6mpg
What else have we found? Perhaps not too surprisingly, our data shows that the most economical cars are hybrids, followed closely by diesels and petrol-powered cars.
However, youll need to check True MPG before you buy from our selection of test engines, the average real-world shortfall for hybrids from the Government figure is almost a quarter (-24.6%).
Petrol cars, meanwhile, tend to perform much closer to their Government figure. The most efficient weve tested so far is the Skoda Citigo 1.0-litre Greentech, which has a True MPG of 53.3mpg that's more efficient than the Government average for petrol cars.
Transmission plays a part in fuel efficiency, too. Most people would assume that cars with automatic gearboxes use more fuel than manual counterparts, and in general our testing confirms this.
However, the difference between the two transmissions isnt as big as youd imagine. Average Government figures show that of the cars weve tested manuals have a 6.9mpg advantage over autos. True MPG, however, shows this edge to be just 3mpg.
Autos are catching up then, and the second most fuel-efficient car weve tested to date is the automatic-only Citroen C3 Picasso 1.4 e-HDi 70 Airdream VTR+, with a True MPG of 70.3.
Better in the real world?
As weve already pointed out, there are some cars out there that might not look so promising on paper, but on the road are less thirsty than Government figures state.
The Mazda 3 MPS, for example, consumes almost 10% less fuel than the Government figures would have you believe, while diesel-powered auto versions of the Ford S-Max and Volvo XC90 are also less thirsty than official figures suggest.
So far and were still relatively early on in our testing schedule weve found nine cars that either match or beat their Government figure:
1. Mazda 3 2.3 MPS +9.7%
2. Nissan 370Z 3.7 V6 +6.8%
3. Volvo S60 3.0 T6 AWD 304 auto +4.6%
4. Volvo XC90 2.4 D5 200 auto +3.2%
5. Ford S-Max 2.0 TDCi 163 auto +1.5%
6. Peugeot 5008 2.0 HDi 150 +0.4%
7. Volkswagen California 2.0 TDI 140 0%
8. Toyota GT86 2.0 0%
9. Subaru BRZ 2.0 0%
Whether youre torn between diesel, petrol or hybrid, and a manual or auto gearbox, True MPG can help you see things more clearly.
Our online True MPG tool will even calculate your very own True MPG and tell you how much your next car is likely to cost you at the pumps.
Its little wonder that in just 12 months, 1 million cars have been searched for on True MPG.
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