Yup, Jag engine. I normally get about 675 kms to a tank but out on the open road, it goes way up. I don't typically drive over 100 km (60 mph) as I travel mainly back roads.
i think you just got a good tank of fuel, maybe ethanol free, without realizing it.... I routinely get 5-10% better on pure gas and especially if traveling at your speed mentioned.