MPG decrease with new wheels

If he goes with the 205/50/17 the diameter will actually be slightly larger than the OE 15" tire.

 

With all the speculation about tire/wheel size's effect on fuel economy, I find it interesting to note the approach BMW has taken using 155/70R-19 Bridgestone's on their new i3. I suppose the diameter is a styling choice and the narrow width is to enhance efficiency.

 

Sort of and yes.

Larger diameter wheels is the general trend for vehicle styling. The larger diameter also helps maintain a properly sized tread footprint for safe braking and handling. The only way to get around the negatives of large diameter wheels is to make them very narrow to reduce weight and improve aerodynamics. Because the i3 is considered more of a city car, the narrow tires help improve agility at medium to low speeds and reduce aero drag at medium to high speeds.in general larger diameter tires increase efficiency but on the Prius, you suffer losses associated with tire/wheel width, weight, aerodynamic drag, etc.. Unfortunately they don't have a tire available to accomplish what BMW did. I.e. A very narrow 17-18 wheel coupled with a 175 or smaller tire.