assuming the average 18" cast wheel is about 23 lbs, do u think it will affect mpg...if so by how much ?
Yeah, unsprung weight and increased drag will both contribute to lower mpgs...looking around the boards, the 17" scion tc wheels some people are using on their prius' report a 3-5 mpg drop. Tc wheel weight is in the same range as what you're considering
Additionally, You will have trouble finding low rolling resistance tires. Most 17" and larger tires tend to be performance oriented and thus emphasis traction over ease of rolling. While the weight of the wheel will indeed lower your MPG, I feel the tire choice is even more crucial. After the debut of the 2010 Prius there are a few decent 17" fuel efficient tires available but I have not seen any really good ones in a 17" or 18" size yet.
Continental ContiProContact is LRR in a 225/60-18 Michelin Pilot HX MXM4 is LRR in a 235/55-18 I have no experience with either tire or size, I just went to find a 18" LRR tire.
You also have to consider the RPM, (Rotations Per Mile) from factory OEM tire/wheel combo vs what you want to now put on your car. That pecentage of difference will also affect MPG.....
