bit more complex
the car's MPG is not part of the calcs. but it is banded on what you can claim, I think it's about 45p a mile for he first 10k miles then 30p thereafter. Which I think averages to 40p/m (cant be arsed to work it out) but fuel costs have at least doubled since the rate was last changed. which feels a bit like I'm gettign screwed, but then again I still get about twice my petrol costs in mileage, so I cant really grumble, and the other costs of wear and tear, insurance servicing etc are somewhat moot, as if I didnt get the mileage, I'd still run a car... which would need servicing insurance tax etc
also the rules re: company cars have changed a lot too in the same time period.
confusing gov website fails to explain it clearly here.