So. We turned around and drove back to the rental office. We actually called them first and asked for a gas car and they said they didn't have any. So we threatened to just go to Avis (who said they would gladly give us a gas car) and one way or another they somehow "found" a gas car for us, at some extra cost that was comparable to what we would have paid Avis. LESSON LEARNED NEVER BOOK SUPPLIER'S CHOICE