That sort of fuel economy reminds me of my late father's Daimler 'Double Six', a 5.3 litre V12. It had a 'performance computer', which could tell you, amongst other things: how much fuel (in litre or gallons) you had left in the tank; how far you could go before re-filling, and, alarmingly, the instantaneous fuel consumption in mpg.
If you were going up hill it would read 5 - 8 mpg, but when going down hill you might get 25 mpg!

