As noted, some of the design choices made me scratch my head including a few duplicated properties & methods between Car and Engine and FuelTank and FuelTankDisplay. I ended up with an over-engineered solution because I couldn't figure out what the hidden tests wanted. What the heck does it mean for a FuelTankDisplay to be "Complete" for instance? After having seen the other solutions, I understand the problem a bit more and would have gone about almost everything differently. I don't have the fight in me at the moment though.

Anyway, I think this is a great example of deducing requirements from unit tests, which admittedly does happen on occasion irl. However, it's not the best kata for OOP or fundamental structure. Filling in the knowledge gaps in the description could go a long way toward improving this kata and reducing frustration.