I recall that a number of other Japanese RPGs had a policy of making things so arcane and obtuse in places that you'd have no choice but to purchase a guide to navigate some of the otherwise impenetrable hurdles the game would present. I have a feeling things like the Detroit Car Wreckers and the Flirtin' Girl were much the same idea; you might theoretically navigate through it given enough time and patience, but far easier to get the guide.
Interesting! It reminds me of the Sierra Adventure games I grew up on were deliberately obtuse / they'd sell strategy 'hint books' with scratch off clues that would bring you closer to a solution. That was almost as much of a business as the games themselves.
I've been randomly looking at Zero4 Champ related stuff on Japanese Auctions, and haven't come across a strategy guide for Zero4Champ II yet, but I have found some for a few of the sequels.
Like this one for Zero4Champ DooZy-J Type R which I'm really excited to eventually play because... Well... just look at that title! It might take awhile to get there at this rate though... It's three games away.
This unofficial strategy guide looks particularly interesting:
Zeroyon Champ DooZy-J
A book to thoroughly enjoy
This one book will tell you everything!!
Perfect strategies for role-playing events that can't be replicated anywhere else, an introduction to hidden parameters for tuning items that strengthen your own car, and maniacal original illustrations that convey the charm of female characters.
Packed with rare secret tricks to make the game even more enjoyable,
A maniac fan book for playing "Zeroyon Champ DooZy-J" without compromise!
Of course this thing would have fan-art of female characters...
Note the above is from a different Doozy-J strategy guide, but I feel like it gives you an idea what to expect.