I used to get 30mpg out of my old 1.1 Fiesta. Mainly because I drove it like I stole it, but also because 15 year old petrol engines aren't going to be economical anymore.
If I wanted to buy a car for the cheapest possible motoring, it would be a small (<1.5) non-turbo diesel in a small hatch that was 12+ years old.
A 106 1.4 or 1.5D that's that old won't set you back more than a couple of hundred quid in pretty decent nick. Skinny tyres are £20 a corner, the diesel engines/clutches/gearboxes are heavy, over-engineered things to cope with the higher compression and torque etc., and hence last forever. It doesn't even matter if they're leaking oil everywhere, and sould like they're about to blow up...they just keep on going. It's also light, small car, so isn't heavy on it's suspension and bushes last a long time.
Also, they give 50mpg if driven sensibly, and whilst slow (some would say dangerously so), it'll still get you from A to B without getting wet.