Uhm- okay then. I'm not sure where to start, exactly, but this fic could definitely use some more work. For one thing, Earth in the HHGttG 'verse is 'a small, inoffensive blue green planet of hardly any interest.' The original entry for it reads 'mostly harmless,' and even the expanded edition was mostly vacation hotspots in the UK. For another, Ford Prefect, the writer for HHGttG, left the planet in 1978, whereas Ranma starts getting interesting in 1987.
It is the 'Infinite Improbability Drive,' not the 'Implausibility Drive.'
The Earth was created by the Magratheans as a gigantic supercomputer - it was occasionally mistaken for a planet due to its use of biological components, and the fact that a boatload of Golgafrinchans accidentally colonized it and wiped out the previous inhabitants.
There's also some definite characterization issues, from Marvin himself all the way through Ranma, Kasumi, Nabiki, and Hinako, making stops at Genma and Soun on the way.