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Economical with the truth? Ten myths about mpg figures

Think you understand those 'official' consumption stats? Think again. Paul Horrell explains

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While choosing your next car, you may think (or think you ought to think) about fuel consumption. Sadly, everything you know is either wrong, or unknowable. Here are ten great economy myths…

1. Manufacturers make exaggerated economy claims. It’s not their claims. The tests are conducted in independent labs, to a Government-mandated drive cycle. Those are the only figures they’re allowed to quote.

2. It’s an independent test, so the manufacturers can’t cheat. Depends on how you define ‘cheat’. It’s gaming the system, but it’s allowed. Mostly, they set up their powertrains to do well at test speeds not real speeds, and test them with the Eco button pressed, making the car so sluggish no one would use it that way. Plus they can accumulate all the little test tolerances in their favour. And they test with tyres pumped dangerously hard.

3. The official tests are at least mimicking the real world – urban and extra-urban driving. Not the real world we live in. The acceleration is so preposterously gentle and the speeds so glacial, you and I will never drive like that. Also the test doesn’t have any element of motorway-type driving. Think you mostly drive in town? Think again. Say you drive an hour in London, then just half an hour on the motorway. You just did 10 urban miles and 40 motorway miles, so the motorway bit is what matters.

4. The CO2 result is separate. Nope. The CO2 is proportional to the fuel used – although there are two different constants of multiplication, one for petrol and one for diesel. The test doesn’t even measure consumption. It measures the gases in the exhaust and calculates consumption from there. So your actual CO2 output is worse than the official rating. The rated CO2 figure matters because it affects VED and company car tax. Real fuel consumption matters because you have to buy the stuff. Real CO2 and consumption matter to resource depletion and climate change. (For completeness, note: at or near full throttle and outside the test cycle, proportionality breaks down because many turbo engines are programmed to throw extra fuel into the cylinders to cool the pistons, which just washes out of the exhaust.)

5. Small turbo petrol engines are much more economical than equivalent-power unblown big ones. Hardly. They do well in the official test because during that cycle they never come on boost. If you extract the performance they’re capable of, consumption plummets towards that of the naturally aspirated engines of a few years ago.

6. Hybrid powertrains are economical. Not on the motorway, where there’s no recuperation to speak of, so they don’t do especially better than anything else.

7. Plug-in hybrids are sensationally economical. Don’t make me laugh. They take the same unrealistic test all cars go through, but with the added wrinkle that they’re allowed to deplete their battery. That electricity isn’t stated, so they’re driving partly on ‘free’ energy. If you plug in your hybrid after every short trip, fair enough. But beyond the 15-ish miles of electric range, it will just act like any other hybrid, or worse.

8. Congestion is the enemy of economy. Not on the motorway. You’ll probably get half as far again on a gallon if your speedo says 55mph on a busy but free-moving motorway, compared with the 80mph you’d otherwise choose. For instance, that’s 60mpg versus 40mpg. And the gradient for small turbo petrols is steepest of all.

9. You can get the truth from those websites where people submit their own mpg. You really believe anonymous internet posters? You don’t know how they drive, or whether they’re submitting their (usually optimistic) trip computer figure or a real number calculated from recording fuel fills and corrected odometer readings. Or if they’ve got a grudge, or work for rival manufacturers.

10. In normal driving, all petrol superminis do 40mpg and all diesel superminis do 50mpg. All mid-size hatchbacks are 5mpg worse than that, bigger cars another 5mpg down. Subtract 5mpg for hot hatches. Crossovers are 5–10mpg worse than cars the same size. All supercars return about 15mpg, except for 911s which do 25mpg. This is actually true.

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