Wheels & Tires - c6 17x9.5 wheels from OE wheels

I'm asking the question for a friend with a v6 fiebird. The car has the 16" snowflakes with 225s from the factory.

He brought up something I couldn't answer, but the question is does having a bigger wheel 17 or 18s with smaller sidewall kill his mpgs significantly.

Also what is the "minimum" tire can he put on 17x9.5 wheels since he doesn't really need 275s.

Thanks

275/40-17 is the proper size tire for a 17" wheel on a 4th gen car. It will have the same diameter as stock 16" tires, so your mileage won't change. Any slight changes would be due to weight differences in the wheel/tire package.
What year Firebird and what was the original tire size?

You can run a 245-255 tire on it. They just don't look near as good!