Arena is easy - those weren't parsecs that Sulu was counting off during the chase, they were partial sectors (AKA par-sects)It is also one of the most workable speeds for the ship for the purposes of star adventures. That is, assuming that it corresponds to "warp 7" rather than to "maximum velocity". Which is basically just a matter of Kirk taking more risks than Pike. Or Scotty more than, uh, Pitcairn or whoever.
You can't account for quoted feats like "Arena" or "That Which Survives" with .73 ly/h. But you can quite plausibly have interstellar adventures with that!
Timo Saloniemi
