New advisory mileage rates for tax purposes came out last month. http://www.hmrc.gov.uk/cars/advisory_fuel_current.htm

It seems to me crazy that we still give higher rates for bigger cars. If we switched to a flat rate (somewhere in the middle of the current spectrum) it would give a real incentive for people to buy the most efficient cars possible. It would also be progressive, as it would reward people on lower salaries who can only afford a smaller car.