The old Centurion Tank,
(Mk 12 version, 47 tons, powered by a 1650cid V12 Rover Meteor Mk 4B petrol engine).
was officially rated for 0.5 mpg on the road and 0.2 mpg x-country.
We used to go from one side of Salisbury Plain to the other (~40miles with most of it Artillery Impact Area i.e. full of craters, which made it pretty slow going) and the 220gallon tanks weren't far off empty.
If that wasn't exciting enough, re-fuelling them by hand with 4.5 gallon jerry-cans was even more fun with almost red-hot silencer boxes on each side at the rear right below the filler necks igniting splashed fuel from the tank crew trying to get things done quickly with warm, sometimes overfilled j-cans that when opened blew fuel under pressure out of the top, this was sometimes all done in darkness, which made it even more alarming.
Good waste of fuel though. Every minute was worth a lifetime.