A Ford KA does about 45.6 miles per gallon. You did 83 miles. That means you used 1.82 gallons. In litres this is 8.27ltrs. Petrol costs about 95p a litre. So based on that, the journey cost £7.86. If you want to divide that by the two of you, it would be £3.93.
Arnt you a good friend. I wish...