As an engineer, I know where this comes from.... And in older cars, yes this is somewhat true. In an older car, the fuel systems were very, I mean VERY basic. They were actually exactly what you would imagine in your head right now. A simple fuel tank with a spout, or filler hose, that led up to an accessible hole where fuel could be added. In this gas tank was a basic fuel pump, which was somewhat easily accessible for a reason (always had to be replaced). Things were even worse before EFI came about. Anyways, back on topic. Fuel tends to evaporate relatively quick. Therefore, this claim has to do with the rate the fuel would actually evaporate from the tank whether it was actually in use, or just sitting over night. A smaller amount of fuel (i.e. 1/4 tank would obviously evaporate faster than a full tank, due to surface area etc). New cars have a lot more advanced fuel systems. They eliminated nearly all the evaporation issues (from the little flap on the filler spout to advanced gas caps, circulation hoses .. the list goes on). But thats how that theory began and people have tended to continue believing it. Trust me, you waste more money by making more frequent stops to refuel due to only filling up halfway, rather than filling up and not having to stop as often.
