In fact while we're on topic I had a conversation with my neighbour about this the other week.
He says that I should pay "Gas guzzler" tax cos I own a Jag (2.5 litre X Type). When I pointed out that he does 3 times more miles in a year in his Astra Diesel than me, & therefore 1) Guzzles more fuel than I do, & 2) uses the roads more than I do he went a little quiet................