One of the most common tuning upgrades for compact cars used for everyday-driving is an improvement of the car’s fuel efficiency—this is usually done through electronically remapping the engine control unit and modifying engine characteristics.
Of course, the Toyota Prius already has stellar fuel economy, so if you aren’t in need of a horsepower boost or more torque, tuning your Prius may not be worth the time and money you’d put into it.