The Polo BlueMotion is one of the first of a new type of small car to arrive on the market. All of them are being promoted by their manufacturers as having CO2 emissions under the 120g/km barrier which will - if current proposals are confirmed - make them ineligible for the revised London congestion charge, and several of them, like the version of the BlueMotion tested here, have a two-figure CO2 rating which means they are also zero-rated for Vehicle Excise Duty.As explained in our Fuel Economy Testing feature, low CO2 emissions are the same thing as low fuel consumption, so good economy is an added benefit, though it's notable that the manufacturers are generally drawing more attention to the tax situation.The BlueMotion programme involves what is rapidly becoming the standard method of reducing CO2 figures. Volkswagen has taken the regular Polo 1.4 TDI turbo diesel, done a moderate amount of fiddling with the engine's ECU, raised the gearing in third, fourth and fifth gears, made several aerodynamic changes (most obviously a deeper front bumper and a smaller radiator intake, though there's some underbody work too) and fitted tyres with relatively low rolling resistance.There are enough checks and balances here to leave the performance more or less unaffected. In terms of fuel economy and CO2 emissions, though, the improvements are startling. Combined fuel economy is now around 74mpg, and carbon dioxide is produced at an official rate of 99g/km.Or at least that's the way things are with the BlueMotion 1 referred to here. The BlueMotion 2 is less economical (though it still manages 70mpg on the combined cycle) and although it will still dodge the proposed London congestion charge its 104g/km CO2 rating means you'll have to pay VED, though admittedly not very much. Despite this, Volkswagen reckons it will still sell two 2s for every one 1 it puts on the UK market.The reason for that is that the BlueMotion 1 is just that little bit more basic than most UK buyers are likely to stomach. The BlueMotion 2 has air-conditioning, a "coming home" headlight function, automatic wipers, dimming rear view mirror, some leather trim, remote central locking, an alarm and a proper spare wheel, none of which is available on the 1.Although their presence on the 2 doesn't make that car much heavier, there is enough of a difference for it to be fractionally less economical and therefore perform not quite so well in the official economy test.Even so, both cars are memorably frugal, so you wouldn't spend much time visiting service stations in either of them. Since they use the same engine - already quite familiar from its use in other Polos - they also share a couple of other features, one of which is that they are surprisingly quick. A top speed of 109mph and 0-62mph in 12.8 seconds may not sound very interesting, but those statistics do nothing to convey the BlueMotion's sturdy performance during mid-gear acceleration.It's also an exceptionally good motorway cruiser. The gearing may have been raised (by a spectaular 25% in top) but the little TDI has absolutely no problem with this, and will keep up with virtually all other traffic while chuntering along at very low engine speeds and with no need for a precautionary dip into fourth gear.The only downside - and this is common to all 1.4 TDI Polos - is that the engine is very noisy indeed. The three-cylinder thrum isn't particularly offensive; it's just that there is an awful lot of it, whether you're accelerating hard or sitting waiting for the traffic lights to turn green.This is the worst thing about a car which is very appealing in all other respects, and not just those relating to giving money to fuel companies or HM Treasury. All Polos are good to drive and the BlueMotion is no exception. I must admit I was slightly concerned when I first heard about the low rolling resistance tyres, but having given the BlueMotion a bit of stick (entirely in the interests of research, you understand) I'm now happy that it has enough grip to be able to cope with anything that most drivers will throw at it.Despite all these positives, the financial considerations which are supposed to be at the heart of the whole BlueMotion philosophy don't really add up. The entry-level car in the line-up, the BlueMotion 1 three-door considered here, costs several hundred pounds more than the cheapest regular Polo with the same engine, and it's going to be a long, long time before you make that back by not paying VED. There's also no potential London congestion charge benefit, since other 1.4 TDI Polos are also below the crucial 120g/km emissions level.At the top of the range, the BlueMotion 2 five-door costs £13,445, which is a lot of money for a car like this (the typical Volkswagen build quality makes it seem more justifiable, the thundering of the engine less so). There are certainly benefits from the improved fuel economy, but they will be most apparent to drivers who rack up the most mileage each year.The most important thing about the BlueMotion is that it has shown what Volkswagen can do with the Polo to improve its fuel economy and reduce the tax burden on its owners, and I suspect that its legacy will become clear when the methods of doing these things become standard on all Polos, rather than just on a limited range of expensive models. Engine 1422cc, 4 cylinders Power 79bhp Transmission 5-speed manual Fuel/CO2 74.3mpg / 99g/km Acceleration 0-62mph: 12.8 seconds Top speed 109mph Price £11,995 Details correct at publication date