Diesel is £2 a litre, Luckily my car is economical - 67 mpg.
If you want to convert to pence per mile - then take the cost of fuel (in pence), multiply it by 4.54 and divide by your cars mpg.
(200 *4.54 / 67)
(Obviously mpg can vary when in towns vs long drives )
30 mpg @£2 a litre is 30p
60 mpg @2 a litre is 15p
You can see how journeys can add up - and how some people who have to drive for work are struggling.