There's really no need to make some childish or cringy car transformation just because it's a mecha skin. He's already a cat, there's nothing he should transform into. Furhermore, Mecha Kha'Zix was first mecha skin released, and he doesn't transform into anything, which means that there was no intention that every champ within this skin line should transform into something.
IMO perfect recall for this skin could be that by pressing B, Rengar leans down (like he does when you enter the bush and stop moving) scans the ground and finds the trace of Kha'zix, then displays his blueprint in front of him (like in the splash art), then he hears something (Kha'zix), turns around his head, quickly pulls out his blade (like Wolverine does with his claws) and then digitally disappears/turns invisible.
Such recall fits Rengar theme and his lore, because he is a hunter, not some kind of taxi driver in the jungle. At the moment, the transformation into a car does not make any sense and doesn't fit Rengar overall, and it feels like designer team just were out of ideas what to do about his recall, that's why the recall is so out of place/random. If Rengar transformed into a car during ultimate, then yes, it would make sense, but now, it just doesn't.
P.S. Would be grateful if someone posted this idea in pbe mecha rengar feedback thread, because i have no access to pbe.