Hello,
We rented a SUV last August via what appears to be an agency with good reviews- I think it was Zanzibar Best? All contact was through a guy named Ahmed who delivered our car. He arranged our local driving license. We resort-hopped on different beaches for a couple of weeks and it was nice to have transportation handy. It was a little tricky on the narrow side streets in Stone Town and I was glad my husband was driving, but navigating long distances across the island was fine. Some roads are great, some not so much. We are going back in June and are renting from Ahmed again but I just contacted him through FB. I don't remember the price of gas, but it didn't seem expensive. Let me know if you have more questions.