Not sure about Dover
But if any use for your info we run on an overnight ferry route of over 180 miles and if using 2 (out of our 4) engines burn about 20000 litres over a 12 hour period. Average speed of around 16 Knots.
wonder if the captain gets clubcard points or do they still do that offer of a voucher for glasses and decanters for every five quids worth?
i used to fill up the lorries at my depot and the guy at esso used to give me a box of glasses every week,easier than counting out vouchers!