I’d like to suggest that you actually don’t need to complicate it so much.
All you need to do is to use a single engine loop sound and use Continuous Modular to adjust pitch.
From the Actor’s blueprint just feed the RPM / MaxRPM to the float parameter. I’ve tried this and it works much more elegantly.
Gear shifts are automatically handled gracefully because RPM drops when gear shifts. It sounded really natural because real car sound is tied to the RPM of the engine.