As of April 2009, UK Road tax for new vehicles is calculated on CO2 emissions. The rules and prices for this calculation can be found below:

Cars registered after 1 March 2001 (based on fuel type and CO2 emissions)

Petrol and Diesel Cars
Alternative Fuel Cars
Band CO2 (g/km) 2014
First Year Tax
2014 Band 2014
First Year Tax
2014
Up to 100 0 0 0 0
101-110 0 £20 0 10
111-120 0 £30 0 20
121-130 0 £110 0 100
131-140 £130 £130 £120 £120
141-150 £145 £145 £135 £135
151-165
£180 £180 £170 £170
165-175 £290 £205 £280 £195
176-185 £345 £225 £335 £215
186-200 £485 £265 £475 £255
201-225 £635 £285 £625 £275
* 226-255 £860 £485 £850 £475
* Over 255 £1090 £500 £1080 £490

Cars registered before 1 March 2001 (based on engine size)

 
Private / Light Goods Vehicles
  Engine size All Car Types 2012-2013
  Not over 1549cc £135
  Over 1549cc £220