Ford Model t MPG
From what I've heard, current owners get around 25 to 35 mpg (US) on modern fuel after retuning to take better advantage of it (greater ignition advance, etc). Which compares favorably against a typical large SUV or pickup truck, and isn't much worse than a lot of mediocre econoboxes... But when you consider that these figures are achieved with a cruising speed in the 30 to 40mph range and an engine that soon shows its weaknesses when pointed at a hill (20 bhp really isn't that much, and you only get that for a limited portion of the rev range that the wide ratio 2-speed transmission means you're usually outside of), it doesn't sound so good any more. The T had a pretty primitive engine even for the time after all, and basically nothing in the way of aerodynamics.
For comparison, in an early 90s base-spec European hatchback with more than twice the power and top speed, i could get 60+ mpg US if I kept my cruising speed right down to T levels... But then maintain it up even the steepest hill (seriously, i never found one it couldn't crest at more than 40, fully laden)... And about that much at 50mph in a twice-as-powerful-again turbodiesel (needing to drive it basically flat out, up into the 110mph range, to see T-like economy).
On the poorer quality and more variable fuels available during its heyday, the power output would be rather less guaranteed and the economy markedly worse because of the need for the engine to run in a relatively detuned state to avoid shaking itself to pieces through predetonation - or in fact to keep running at all... To say nothing of literally running it on moonshine and the like...