When we went, it was cheaper to disconnect the car and pay for that separately.
We took the short ferry up, and the long one back, mostly because of time...
It's 500mi from Port Au Basque over to St. John. On the West coast (PoB) there are TONS of things to see, and in our opinion, not to be missed. Wished we'd been able to spend a week just in that area. You can go to the most Northwestern tip and see one of the original Norse settlements. Tons of history up there, and every where else.
We were there for the month of Sept. Weather was gorgeous, never had trouble finding a place to stay or finding fuel. Paying for it?...yeah, that was hard ;)...but finding it was easy.
Driving across the Rock, be prepared for a constant stream of "OOOohhhhhhhhhh!" and "AAAHHHHHHhhhhhhhhhhhhhh!"s coming from you and your mate. Every corner, every hill you crest, will present yet another incredibly scenic vista.
Jim