Lets knock this £18% thing on the head: The % will range from 15% (for a small petrol engined car), up to a maximum of 35% and is entirely dependant on the model chosen.

Every car sold in the UK has an official CO2 grammes per Kilometre figure, and this decides the % of the List Price When New that you pay tax on.

From 1/1/06 the 3% supplement for Euro 4 compliant Diesel engined cars has been reinstated, as - effectively - it is now illegal to sell a non-Euro 4 car. (The exception is if the car was built before October 2005, it can still be sold and registered up to the end of this year)

The HM Revenue & Customs website has an easy to use calculator, but you need to know the figures applicable to your car when new:
http://www.hmrc.gov.uk/calcs/cars.htm

Taking a standard new-model Focus 1.6 LX 100PS manual (need the exact model to be specific, because even something like different wheels/tyres can affect the CO2 rate), the current list Price is £13,395 and the CO2 figure is 184g/km. That means your taxable BiK is £3080.85(£13,395 x 23%) so the car actually costs a 22% taxpayer £677.78 or £1232.34for a 40% taxpayer.

The fuel benefit is entirely separate, and the current scale charge for ANY private fuel being provided is £14,400 to which you apply the same % as calculated for the car benefit, in this case 23%, so you pay tax on £3312 and meaning the "free" fuel costs our 22% taxpayer £728.64 or £1324.80 for 40% payers.

And I do this as a day job.