cockney steve,
4/6 a galllon, forsooth! In my young days it was 1/4, and ROP undercut it @ 1/3 or 1/2. And I still recall the headlines (just prewar) when 20 ciggies went down from 1/- to 11½d !
I'd quite forgotten the twin-tank marine engine, but that solution was hardly practicable in a road-car.
Our ever-patient Moderators allow considerable latitude on this Thread (this has made it into the most popular Forum which it is).
Danny.