Sean O'Grady: Saving the planet will kill you, it seems

But then the United States National Highway Traffic Safety Administration has to go and spoil the good clean fun. The agency has received complaints about Priuses stalling or stopping unexpectedly at "highway speeds". In a very few cases, the clever software that turns the petrol engine off on an overrun, say, then fails to start it up again when it's needed. The good news is that the electric motor will still work and look after the power-assisted steering and braking.

The bad news is that, from my experience, I'm not sure how competently the electric motor would take one from the overtaking lane of a busy motorway at 70mph to the hard shoulder at zero mph. I've found that the electric motor won't work, on its own that is, at more than 30mph. You can coast to safety on no engine power at all - if there's nothing else on the road. My worry is being caught trying to bear left on our crazy motorways.

I phoned Toyota. I was reassured that all affected Prius drivers will be told to go to a dealer to have the software sorted out. My own Prius was unaffected.

Exemplary behaviour by the maker, you might think. Except that I am left wondering how the software on some cars can vary. And how a Toyota can go wrong. Very odd.

Otherwise, it seems to be a pretty blameless vehicle. It's been impeccably reliable in my 2,000 miles with it, never returning less than 50mpg although rarely exceeding 55mpg, despite official government figures suggesting otherwise. Other readers have told me that 60mpg-plus is perfectly possible and folk on web forums devoted to the Prius confirm that there's a lot of money to be saved there.

But am I missing the point? One reader pulled me up after my last report on the Prius by pointing out that I shouldn't just be worried about my wallet and my fuel bills and whether that justifies the premium price of a Prius. Rather, I should be looking at the transcendent issue of saving the planet. I must admit I'd never seen things like that before. From the point of view of our poor old battered Earth, hybrid cars such as the Prius, its bigger SUV brother, the Lexus 400h, and the Honda Civic IMA cannot really be beaten, (except by public transport), and whether they make you a bit better or worse off is irrelevant. Also of no consequence is whether hybrids are enjoyable to drive or attractive to look at (they aren't). It's all about the real cost of motoring, to the planet, as one reader told me. What do you Prius drivers (and other readers) think? Would you buy one?