My old Audi A4 1.9l TDI 90bhp (1995 N-reg) supposedly emits 125g CO2/km, yet as it was registered prior to 2001, it will cost me £190 to insure. Under the new rules, if it were registered post-2001, I'd have to pay £90 p.a. My question to you is, if it would be possible (and feasible) to re-register the car in the UK to be subject to the new CO2 emission rules (assuming the 125g number is correct, I'm dubious about this) and thus pay less VED tax?

Those CO2 emission numbers are based on tests but in theory, a litre of diesel or petrol should already have a pre-determined amount of hydrocarbons, oxidation of which would yield a predictable amount of CO2. Is there an easy way to calculate how many grammes of CO2 per litre my car is emitting based on its mpg? One number I have found on the web is 2.6kg CO2 per litre. Using school math, when my old 16 year old diesel gives me 45mpg (it actually does, based on my record keeping) I calculate this:

62 litres of diesel yields me 622 miles. Using the 2.6kg number multiplying it by the litres, I have burnt 161.2kg of CO2. Incidentally, 622 miles is a neat 1000km. Therefore, 161.2kg used in 1000km, that's 161.2g CO2. So the question for me is which number is wrong? The 125g CO2 (found on Wikipedia, which used Ultimatespecs.com [http://www.ultimatespecs.com/?option=4&brand=2&model=613&car=3802]) or the 2.6kg CO2 number? Or is it some other reaction which perhaps results in more carbons coming out as something else than CO2? Dust particles perhaps?