A gallon is 4.54609 litres. If petrol costs 83p per litre, thats £3.77 per gallon.
If you divide this by your mpg you get the cost of petrol per mile.
When i had a mk3 16v i averaged 30mpg so thats 12.6 pence per mile
My VR6 averages 26 mpg so thats 14.5 pence per mile
A difference of 1.9 pence per mile
40 miles a day, thats an extra 76p per day.
If you did 20,000 miles in a year it would cost you £380 extra a year in petrol.
Obviously your average consumption will probably be slightly different in both cars, but you get the idea.
And no, of course i'm not bored at work today!