At the moment it's still considered a big deal for a car to emit less than 100g/km of CO2 (on the official EU test cycle, if not necessarily in real life) because of the tax benefits. If you have a car which does this, you don't have to pay Vehicle Excise Duty each year, and you can breeze into London with a happy disregard for the congestion charge.That situation won't last for ever. As sub-100g/km vehicles become more common, the threshold is bound to be brought down, so in order to give customers the same benefits manufacturers are going to have to achieve lower CO2 figures.Ford has reacted to this by improving the green performance of the Fiesta ECOnetic. Until recently, this car was rated at 95g/km, but through various efforts this has been reduced to just 87g/km, making it more likely that the next threshold adjustment will leave it untouched.If you're interested in this sort of thing, the reasons for the improvement include a start/stop system, higher gear ratios, reduced friction inside the 94bhp 1.6-litre diesel engine, adjustments to the electronic control unit which dictates how much fuel is provided in a given situation, regenerative charging (to reduce the drag effect of the alternator), an Eco mode and improved efficiency of the cooling fan, radiator and air-conditioning system. As with the previous ECOnetic, this one also has low rolling-resistance tyres, a variable flow oil pump, a gearshift indicator (which doesn't help in itself but might persuade you to drive more economically) and an oil pump which pumps oil only at the required rate, again reducing drag inside the engine.You might be asking, as I often do, why these things are not standard on all Fiestas. In the case of the ECU work and the gear ratios, for example, there's no obvious reason why they shouldn't be, as neither of them particularly affects the performance of the car.Elsewhere, cost must be a factor. Of the five-door Fiestas in Zetec trim, as tested here, the ECOnetic is the most expensive by £1100, and although it also happens to be the quickest (slightly beating the 1.4 petrol model, which is nearly £2000 cheaper) this may not be your number one reason for choosing it. Not for the first time, here's a low-CO2 special which seems to be a very expensive way of saving money.That, admittedly, is solely in terms of the VED payments. At current rates it would take you 55 years to claim back the premium over the 1.4 diesel. If you're normally a London congestion charge payer you will, of course, save money much more quickly.And then there's the fuel economy question. The ECOnetic's combined figure of 78.5mpg is nearly 10mpg better than that of the 1.4 diesel, which is pretty good. How achievable this is in real life I don't know, because this test was too short to find out. We'll have another ECOnetic for a week in the near future, and you'll be able to read our findings shortly afterwards (see below).If the results are good, I suspect the gearing will have a lot to do with it. At 30mph in fourth gear the engine is burbling over at just 1100rpm. which is not much over the lowest speed it can cope with - it won't accelerate happily from that, but it will do from 200rpm higher, which is pretty good going for a small diesel. 70mph in fifth gear equates to a sniff over 2000rpm, so I imagine the ECOnetic will be quite gentle on fuel on a long motorway journey.In other respects this is a perfectly normal Fiesta, with the usual attributes of an impressively high-quality feel and dismal rear visibility. The ride quality isn't all I'd hoped it would be, considering the car runs on sensible if not particularly attractive 16" wheels, and since no changes have been made to the suspension it seems that those low-rolling resistance tyres are causing the problem. This may be an acceptable price to pay for their contribution to the impressive economy and CO2 figures, but I'd still like the ride to be less jiggly than it is.Update (23/10/12): As mentioned above, though a little later than expected, I did indeed have the ECOnetic for a further week's evaluation. I wasn't planning to drive it more economically than I would any other car, but in a way it's difficult not to.The low-drag nature of the engine and tyres is more noticeable on the Fiesta than it is on many other low-CO2 models. In particular, whatever speed you happen to be doing, you can maintain it on a level road with just the barest pressure on the accelerator pedal. (Within reason, obviously. It's not going to be like that at 100mph. Furthermore, the engine works very well at low revs, so despite the gearing you might find yourself driving the car for days without exceeding 2000rpm.Yes, fine, but what's the effect of all this? Well, I didn't match the official 85.6mpg figure. Nor did I come close to the quite staggering 108.78 achieved with an identical car by former rally driver Andy Dawson in this year's MPG Marathon. But in mostly town and A-road running, sometimes in bad weather, often in traffic, nearly always with the air-conditioning switched on, I did manage 63.8mpg. For me, on the roads near where I live, that's a very good figure for a diesel supermini.I never came to terms with the ride quality, though. And there was a lot of road noise. And too much information about the state of the tarmac was conveyed by vibrations of the steering wheel. And the engine noise seemed more insistent than it does in other diesel Fiestas. This is a lot to be dealing with in a car costing over £15,000, and despite the fuel economy and the tax benefits I can't help thinking that if I were going to buy a Fiesta, it wouldn't be this one. Engine 1560cc, 4 cylinders Power 94bhp Transmission 5-speed manual Fuel/CO2 85.6mpg / 87g/km Acceleration 0-62mph: 12.9 seconds Top speed 111mph Price £15,595 Details correct at publication date
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