There's this idea floating around in my head. I'd like to get it out but I need
some information first.
What is the approximate distance from the Imperial Palace in Tokyo to the
Nerima Ward? Is it under 30 km?
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Boy, that's a good question...  I think the eastern edge of Nerima may be
within that range, if you go as the crow flies.  But if you go by the book
"Ranma no Himitsu", which places the Tendo Dojo in the western half of
Nerima, then they're probably more than 30 km from the Imperial Palace. 
But not by much.  It's hard to tell without a map, because I've only gone
places in Tokyo by train, and its hard to judge distances travelled on
trains.  

However, for purposes of writing a fanfic, I'd say go ahead and assume
that it's less than 30 km.  It's not like most Ranma fics (or even the original
by RT) was terribly realistic anyway.  I think its safe for you to take some
liberties on this point, so long as the resultant fic is good.