Learn Finnish: Tämä toinen auto kuluttaa paljon enemmän.

Toinen is used when referring to a specific alternative, like 'the other one' out of a small, defined group (often two). If you used muu, it would imply a completely different kind of car, or 'other cars' in a more general, open-ended sense.

Doesn't toinen also mean 'second'?

Yes! Toinen acts as both the pronoun for 'other/another' and the ordinal number 'second'. In many contexts, like comparing two vehicles, the meanings overlap perfectly: 'this second car' and 'this other car' refer to the exact same thing.

Why do we have both paljon and enemmän next to each other?

Just like in English, you can use paljon (a lot / much) to emphasize a comparative word. Since enemmän is the comparative form meaning 'more', paljon enemmän simply translates to 'a lot more'.

Could I use enempi instead of enemmän here?

No, because here 'more' describes how the car performs the action of the verb kuluttaa (consumes), so you need the adverb form enemmän. You would use the adjective form enempi if 'more' was directly describing a noun.