Erm.. it's bad.
We used to have a system of say £80 for six months and £160 for a year (was less) but then it changed to engine size.. so little 1100cc engines got away with £100 a year etc. The DeLorean was always going to be top bracket...
Now tax is measured on the Co2 emissions of a vehicle. Clean gets cheap... dirty (more powerful/diesel) gets expensive.
However this only applies to cars REGISTERED after September 2000 (correct me if I'm wrong)... and since my car is a new reg it gets emissions rated. 2.8 V6 is automatically top bracket again! (typical)
Dan