I have recently replaced a 20year old diesel car with a newer one with the same engine size. The new one is quoted at 125gCO2/km and has much less tax than the old one. However, the new one burns a gallon of diesel in 45 miles, the old one went for 55miles on the same volume. How can the new one produce less CO2? I also have a small engined turbo charged Renault with quoted urban cycle economy of 52mpg. This seems to be down the lines of the move to small but high performing engines mentioned in the article. However carefully and smoothly I abide by fuel conservation measures I can't get better than about 40mpg from this vehicle. It seems to burn large amounts of fuel when warming up and hills seem to burn fuel more heavily than in previous vehicles. Where do Renault get their figures from? The fact that its performance still bears comparison with vehicles I owned 35 years ago says we are a long way from the claims in the article.

There are also other inconsistencies. Both my newer cars have low profile wider tyres and have tyre wear rates worse than any previously owned vehicles. They also both make hugely larger amounts of road noise. These are both indications of more energy being wasted - not less, not to mention the greater amount of particulate generation from the lost tyre rubber. Particularly the diesel car I own weighs a more than the previous one even if the interior is smaller. Once agin this is going to decrease the efficiency of the modern vehicle.

Just what is going on?

Manufacturers' MPG figures are determined in standard tests, and it would be an unusual manufacturer who didn't at least bear those test conditions in mind when optimising engine performance curves, gear ratios and the like.

I have the opposite problem - my car is classified as 124g/km, but I achieve somewhere in the region of 109g/km by driving sensibly but normally. CO2 emissions are linearly related to fuel consumption, of course. So I pay £90 or so more tax each year than I should, in my opinion.

The car I refer to above is a supermini with a 1.3l petrol engine. It replaced a large family hatchback with an only slightly underpowered 1.6l petrol engine (it had no power steering, catalytic converter or aircon to hold back what power there was). I would get 40-45mpg in the older car, I now get 50-55mpg, surprisingly little difference. Older cars had far less weight - we pay a price for the various safety and comfort features we now expect as standard. Safety features refer both to an increase in shell rigidity around the cabin, and to airbags, pretensioners and the like.

You are spot on about the tyres as well. I deliberately bought the bottom of the range version of my car, it has ordinary non low-profile tyres. I know someone who has stripped out almost everything from his Vectra 2 litre petrol, including the door cards, rear seats, and sound-proofing foam. He claims to now get over 50mpg simply because his car is so light.

I believe figures are trustworthy, but are utterly dependent on the way it's driven (as we all know). I have a ten year old diesel TDCI ford Focus with 170k miles on the clock, and can regularly get 25% better average MPG than the manufacturers figures. Last 50L tankful gave me 850 miles before I filled it up again.

The fact that as a concerned engineer, I nowadays drive like a slug on ice has an awful lot to do with that.

Driving for ultimate economy is the only real bit of "competition" left on the road since student boy-racer days are over and guaranteed to cost a lot or land a fine anyhow. I particularly enjoy the smug ultra-slow acceleration, and gliding down to roundabouts from half a mile out, while those behind rant and rage between accelerator and brake.

Expensive & well inflated "Eco" tyres are WELL worth the money. - Any cyclist will tell you the significant effect that tyres can have, yet with many drivers cocooned from the grunt and feeling of their own tyre resistance it's all too easy to take the cheapo option at the local tyre shop.

There are two things which irritate me about the industry. Firstly, there have been vehicles about for over 60 years capable of 50MPG or more (take Citroen 2CV). It doesn't seem like we have come a very long way in the intervening time, Especially since catastrophic climate change problems are bearing down on civilization.

Secondly, the majority of industry hype still revolves round power and acceleration performance. I know that some manufacturers are beginning (sensibly) to "downsize" engines, but IMO nowhere near enough. Who in the UK really needs 120 PS? I want a diesel family estate with half this power (or less) and 50% better economy. I have a hard enough time as it is keeping the speed and acceleration and breaking DOWN to reduce emissions and to avoid speeding tickets. Sensible cars still just don't exist

It also irritates me that the distance between the official figures and real world driving is growing. Take a look at that list of cars at the end of the article (between 75 and 90mpg). Some of those are worse offenders than others but I know for sure that the TwinAir engine (as characterful as it is) gets nowhere near it's official figures.

One of the main ways in which the manufacturers can "optimise" their figures is with the now more common automatic / automated manual (e.g. DCT/DSG) gearboxes. The tests feature certain speeds and with a manual box a certain gear but with an auto the ECU can be programmed to pull a higher gear than you would in real use to get the best possible figure.

For me though these targets are there to protect the environment and yet we have conflicting schemes such as the car scrappage scheme. This effectively promoted a very environmentally unfriendly act of disposing of a perfectly good car that already exists and replacing it with a brand new one. The emissions from the tail pipe are only one part of the problem, an awful lot is emitted in creating and shipping new vehicles so keeping existing vehicles on the road should be encouraged.