Probably don't really need one as know island is small but have a few specific addresses to find ....... Is a hire car ..... Why? ....... What's wrong with their roads?!
If I remember rightly there was on one decent straight stretch of road and that was only about a ¼ mile long at the most, As R1 says most of the roads are only what we'd consider to be country lanes and the island is that small if you keep driving you'll end up back where you started in half an hour
Forgot to mention but if you're in St Peters Port there used to be a really nice pub on the North Esplanade,right near the marina called the Ship an The Crown, it's a few years since we were last there but you good always get a decent pint and meal there