Only three of these pairs obey one classification. Name the pair that does not.

Three of the pairs join a vehicle to the medium it travels along — the train upon its rails, the ship over the sea and the aircraft through the sky. The odd pair gives the fuel burnt in the tank instead of the way travelled; the road would have completed it. The pair looks at home because petrol belongs to a car as surely as rails belong to a train, but the group is built on where the vehicle goes.