My first job from school was in a factory while I waited for my application to progress for the Army.
One of the old boys there was a Moggie nut andnhad several restored and modified examples. He also fancied himself as a bit of an inventor and was coming up with all sorts of devices to improve fuel economy.
Every Friday lunchtime he'd walk up the pub and the rest of us would have a whip round, 50p each, and one of us would nip out with a jerrycan and buy a couple of gallons of petrol which we'd quickly decant into the moggie.
Fortunately the fuel gauge was too vague to give the game away, but when it came to brim-to-brim fill ups and the following maths the owner discovered he was getting over 70MPG. He was excitedly writing letters to New Scientist, Tomorrow's World, anyone who he thought might listen, while we tittered behind his back.