No matter what your views may be on the environmental debate, it becomes an inescapable factor when you start choosing your next car. The fact that Vehicle Excise Duty is based on various grades of CO2 emissions, the issue is nowadays partly a financial one, and although I've heard wildly differing views about the proportions involved, it must be true that at least some - perhaps most - car buyers will be attracted to low-CO2 cars because of the corresponding reduction in taxation.In this way manufacturers, themselves under political pressure to reduce their overall CO2 output, can gain a marketing advantage by building cleaner cars; hence, among other examples, the existence of Volkswagen's BlueMotion vehicles, of which the Polo tested here is one.Volkswagen has taken the 1.4-litre Polo turbo diesel and BlueMotioned it by adjusting the characteristics of the engine, raising the ratios of the top three gears, improving the airflow over and underneath the car and fitting low-resistance tyres. The result, in the case of the BlueMotion 1, is a CO2 rating of 99g/km, which means that owners do not have to pay VED.But the BlueMotion 1 is sparsely-equipped, so Volkswagen also provides the BlueMotion 2 tested here, and expects this to be the more popular version by a large margin. This one is fitted with air-conditioning, automatic wipers, a dimming rear view mirror, part-leather trim, "coming home" headlights and a full-sized spare wheel. The weight gain over the 1 is marginal, but it's enough to bring the CO2 rating up to 104g/km.The difference in VED, until 31 March 2010 at the earliest, is just £35 per year. In fact, the BlueMotion 2 falls into the same tax band as the standard 1.4 TDI, so it has no advantage there. Equally, all Polo 1.4 TDIs, whether BlueMotion or not, will be exempt from the London congestion charge under the proposed new regulations.In terms of tax and congestion charging, then, the BlueMotion 2 offers no advantage at all, simply because the 1.4 TDI's CO2 emissions are already so low. Having said that, though, even the BlueMotion 2 is likely to be significantly cheaper to run because of its greatly superior fuel economy.Let's look a the combined fuel economy figures first. The 1.4 TDI shows up very well here, with 62.8mpg, but this performance is soundly trounced by that of the BlueMotion 2, which manages a remarkable 70.6mpg. The BlueMotion 1 is even more impressive at 74.3mpg, but that doesn't alter the fact that the 2 is an uncommonly frugal little car, at least when you put through the official EU test.However, you may not be entirely convinced by the official figures. The writer of a road test I read the other day in another magazine certainly wasn't; he reckoned that the effect of the higher gearing was simply to make the driver choose a lower gear than normal, which would mean there was no economy benefit at all except in motorway cruising.At first sight it's a reasonable point. The BlueMotion does feel as if its transmission has been fitted with first, second, fourth, fifth and sixth gears, and occasionally you find yourself using a gear down from normal. But not always, or at least not necessarily. The best way of driving this car is to use the highest gear that still gives you an engine speed of not less than 1500rpm. This is quite easy to do, partly because the little TDI engine performs so well down there, and partly because it doesn't perform a great deal better if you give it more revs.Driven that way, the BlueMotion never felt slow, but at the same time it never used much fuel either. I didn't quite match the official 70.6mpg figure, but on one long run I managed 69.4 according to the trip computer, and I did not achieve less than 50mpg on any journey. This is impressive stuff even for a small diesel hatchback.On the other hand, buying a Polo BlueMotion is a very expensive way of saving money. In the three-door form as tested here, it costs £12,845 (the cheapest car costs £11,995 and the five-door 2 comes in at £13,445). Now, fair play - the Polo is a lovely car to drive, and the build quality is superb, but you could buy a brand new Ford Focus for less, and there are lots of very appealing cars on the market on sale for four-figure sums.The interior, nicely bolted together as it is, seems like it belongs to a much cheaper car, and although Volkswagen quotes remote central locking as part of the 2's extra equipment, the test car certainly didn't have it. Which came as a big surprise. Hell, if I'm paying £12,845 for a small diesel hatchback, I want someone to open the door for me and say, "Have a nice day, sir," as I'm climbing in. I don't want to have to stick the key in the lock myself, even if there is a little torchlight in the fob to help me do it at night.For that money I also object to the interior noise, which is at near-ASBO levels. 1.4 TDI Polos have always allowed way too much engine clatter into the cabin, and if you get up to any kind of speed on a below-average road surface the roar from the tyres is enough to have you clenching your teeth and muttering, "Shut up. Just shut up!"This wouldn't matter quite so much if the BlueMotion were cheaper than it is, and that really sums up the car. I enjoyed having it, and I got a buzz from checking the economy setting on the trip computer, but my heart sank every time I remembered the price.If you have the money to spare and you're looking for a good, economical second or third car, the BlueMotion is probably the car for you. On the other hand, if you have a budget of around £13,000 available for the single car in your household, you'll probably want to look elsewhere. Engine 1422cc, 4 cylinders Power 79bhp Transmission 5-speed manual Fuel/CO2 70.6mpg / 104g/km Acceleration 0-62mph: 12.8 seconds Top speed 109mph Price £12,845 Details correct at publication date