looking over my notes, the farthest I went on a tank this summer was 265 km(166 miles). That included 48 km(30 miles) on reserve. I put in 15 Litres(3.9us gal). Figured on that fillup only that would be 5.7L/100km or 42 miles/US gallon, 53 miles/imperial gallon.
I seem to recall doing slightly better than that one warm mississippi evening but I also remember the tune from deliverance playing in the background as I poked along back roads at 30mph in top gear looking for an open station.
