From 1st March 2001 the annual car tax (or Vehicle Excise Duty - VED) rates are determined by CO2 emissions and type of fuel used. This information is on the car’s log book - also called a V5 document.
If your car was registered before the 1st March 2001 then car tax is based only on the engine size, with one rate for engine sizes up to and including 1549cc and one for engines over 1549cc.