Thread: Which case would this be in...

Originally Posted by Pippynip

Я еду домой by car.

Would that be in the instrumental case or the prepositional?

Here it is prepositional:
Я еду домой в/на машине.
Here both prepositions are possible with some difference: "Я еду домой в машине" means more like "I am moving towards home inside the car", while "на машине" = "by car".

Where are you right now? - Где ты сейчас? - Я еду домой в машине.
How do you get home usually? - Как ты обычно добираешься домой? - Я езжу домой на машине.

Though sometimes in such constructions instrumental is also possible: "Я добираюсь домой автобусом." But на+Prepositional is preferable anyway.