When all the kerfuffle about CO2 exhaust emissions started it would have been almost beyond comprehension that any car could have been given an official rating of just 98g/km, and if you had suggested such a thing you would probably have inspired a vision among your listeners of some tiny, piddly-engined city car which would be unlikely to accelerate from rest to anything greater than walking speed before next Thursday.Well, it's a funny old world, as they say. Look at this: a Ford Fiesta (quite a large car in its latest iteration) with a 1.6-litre engine, and blow me, here it comes with that 98g/km rating which would have seemed the stuff of dreams as recently as ten years ago.It's called the ECOnetic, and if you've been paying attention to how manufacturers go about creating low-CO2 versions of existing cars you won't be surprised to learn what Ford has been up to here. But here goes anyway: the ECOnetic uses the same 1.6 TDCi turbo diesel engine also found in other Fiestas (modified somewhat for increased fuel efficiency) along with higher gearing, a few aerodynamic aids, the same suspension set-up as the more obviously sporty Zetec S (for lower ride height and therefore reduced wind resistance) and low-rolling resistance tyres.Before we look at what this does to the fuel economy, exhaust emissions and tax situation, it's worth considering what effect some of these modifications have on the Fiesta's behaviour. Those tyres, for example, are 65-section jobs on 14" wheels, and speaking as someone whose experience of the current Fiestas has largely involved 16" wheels with low-profile tyres I have to say that the ECOnetic comes as a welcome change. Its wheels and tyres may not look very exciting, but they have a tremendously beneficial effect on the ride quality, which is now verging on the acceptable.It's not quite there, though. Even on sensible rubber, the Fiesta doesn't ride as comfortably as it should, and I suspect this is because most of the development work was done in Germany, where the roads are far superior to the ones we have to use. On very smooth tarmac it's terrific, but on more typical UK highways it's too jiggly.However, full marks for the handling, which is exceptional by most supermini standards and highly impressive by Fiesta ones. The lower ride height may have been introduced for aerodynamic reasons, but it also gives the ECOnetic more lateral grip than it might otherwise have had (for reasons explained in our feature on grip) and allows you to tackle corners in quite a rumbustious manner - or, if we're being sober about this, gives you a greater safety margin at more acceptable road speeds.There's power to match the handling, too. This may come as a surprise for a car which has been tuned for economy, and it's true that the ECOnetic accelerates slightly slower than a more conventional 1.6 diesel Fiesta (though the lower drag means it has a higher top speed). But the engine pulls so well from so low down the rev range - by which I mean well under 1500rpm - that it hardly ever feels as if it's struggling, despite having higher gearing to contend with.Well, anyway. The real point about the ECOnetic is of course the 98g/km CO2 rating, which equates to combined fuel economy of 76.3mpg. These are mightily impressive figures, but I should say that if the trip computer is accurate (and previous Ford experience suggests it will be optimistic, but not by a margin worth complaining about) I managed a rather less otherworldly 54mpg during this test.And yes, that's partly because I spent quite a lot of time finding out how much fun the car is to drive, but even during the first couple of days, when I drove a lot more economically, I still couldn't sustain an average much above 60mpg. 76.3mpg may be what the car records on the official EU test, and I dare say you could match that if you were very careful indeed, but as an everday figure it's just not realistic.If you're not doing 76.3mpg then you're not emitting CO2 at a rate as low as 98g/km either, so let's not get too pleased with ourselves about green issues. Since cars are taxed in the UK according to how much CO2 they emit in test conditions rather than after you've bought them, the 98g/km figure is still relevant to owners, who will probably be quite pleased that their car is exempt from VED and is likely to remain so for the foreseeable future.But how much of a benefit is this? Other 1.6 diesel Fiestas are rated at 110g/km, so they're not tax-exempt, but the VED payments are only £35 per year, and they'll be going down to £20 from next April. How satisfied are you going to be with yourself when you realise you've paid the Chancellor £20 less than you might otherwise have done?It's customary at this point to put forward an argument that a high-economy, low-tax, low-CO2 car is either negligibly cheaper or in some cases actually more expensive to own than a standard model of the same type (which is certainly the case with, for example, the Focus ECOnetic).In fact that doesn't apply here, because the Fiesta ECOnetic is actually the cheapest 1.6 diesel in the range, on account of the engine being available only with the higher Zetec and Titanium trim levels.The ECOnetic is more basic than either of those cars - it doesn't, for example, have air-conditioning, though I found myself wishing it did when I climbed in on the hottest day of the year so far - so it's £700 cheaper than the Zetec. As previously discussed, you'll save an almost negligible amount of money on tax, and you'll refuel at slightly longer intervals, but it's that lower list price, rather than either the running costs or the tree-hugging element, which makes the ECOnetic the most sensible Fiesta of its type to buy. Engine 1560cc, 4 cylinders Power 90bhp Transmission 5-speed manual Fuel/CO2 76.3mpg / 98g/km