Green’s Dictionary of Slang

rubby-dubby n.

[? echoic of its underpowered engine]

(Aus.) a small, under-powered car.

[Aus]‘Banjo’ Paterson ‘Three Elephant Power’ in Three Elephant Power 3: One of these little eight-horse rubby-dubbys that go strugglin’ up ’ills with a death-rattle in its throat, and all the people in buggies passin’ it.