buying a prius ...
there are no add-ons to increase mpg or performance. mostly we lose mpg due to bigger tires and lifts once you start modding.
Don't believe what they tell you about not improving fuel efficiency.
The following will improve performance and MPG.
1 - Switch to a quality electric fan
2 - Lightweight FlyWheel if you have a manual
3 - Lightweight forged aluminum Rims.
4 - LT - HWY tires on those lightweight rims.
5 - CF or Aluminum Drive shaft
Keep your heavy stock rims loaded up with some good AT/MT tires for offroading and your DD tires on the aluminum street rims.
Rotating mass is the killer of Fuel efficiency and power. By reducing that you will notice significant gains.