They changed how road tax is calculated in 2001. I'm unsure of the month but at some point in 2001 the tax you pay is based on the CO2 emissions of your veichle, previously it was based on engine size.

Basically, your car will be based on emissions while your friends will be based on engine size (pre 2001 only has 2 groups... believe it's under 1.6 and over 1.6)

So if you wanted, you could buy a 2000 plate 6 litre V12 and pay less than you do now...