On Portsmouth, you don't. That's why it is necessary to fill up, before going over. We've never had an issue with a 3- 4 day stay, on a full tank. About the only thing you can buy on the island is ice. It's worth it! If you love natural beauty and serenity. And driving on the beach and sand covered trails. Go through the salt water marsh out to the old village. Just be prepared for mosquitos, in the summer, on that side of the island!how did you take gas?