Bruno, I have been lucky enough to see a few rally cars up close. Mainly the WRC Focus. The crew chief admitted that the only original parts left on these things are a few bits of glass (lights), dash peices and the chasis. But I'd say that's pretty significant. Because when you see a WRC race, look at the interior shots. The dash is the same in the WRC Focus as it was in my Focus and same with the WRC Impreza and my Impreza WRX. This also means that the rally cars have the same wheelbase, real working doors, etc. Sure, the transmission is changed (the Focus can't get AWD without tens of thousands of dollars in mods) there is about a mile of pipe welded in for rollcage, the engine only shares the block, etc etc. But I've seen photos of the factories (Prodrive, Cosworth) where they actually take cars off the assembly line and spend months changing them. Subaru even did a piece on the US SCCA rally cars and how similar they are to the consumer cars.

Now, all that said, I'm smart enough to know that I can't take my WRX off road and expect the same toughness or performance. If EVERYONE knew that, maybe my insurance would stop going up becuae of all the boy racers crashing their rides!