Upton... ya got it backwards. They'd be narrower.
to OP... Are you talking about the wheel or the actual tire? I can tell you right now your speedo will be off by a little. You're going to be running narrower tire, so I guess "TECHNICALLY" it might be a little unsafe, but unless you're doing Autocross or just drive like a friggen maniac, you probably should be fine.
If you want a definative answer, talk to a tire specialist.