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The Performance Index (commonly abbreviated as P.I. or PI) is a car performance measuring system that has been used since Forza Motorsport 2.
The Performance Index displays a car's overall performance value, determined by five factors rated from 0.0 to 10.0 (with a minimum of 3.0) and usually influenced by weight:
- Speed: How fast the vehicle can go at maximum speed. This is determined by engine power and transmission gearing. Cars that can reach 268 mph (431 km/h) or higher, like the Koenigsegg One:1 and Plymouth GTX Fast & Furious Edition, usually have a Speed rating of 10.0.
- Handling: How well the car can handle corners. This is determined by lateral G forces at 60 mph (97 km/h) and 120 mph (193 km/h). The Formula One cars and hypercars usually have the best handling in the series.
- Acceleration: How quickly the car can accelerate. This is determined by its 0 - 60 mph (97 km/h) and 0 - 100 mph (161 km/h) ratings. AWD cars usually have higher Acceleration values than their RWD counterparts.
- Launch: The capability of a car launching from a standing start efficiently (e.g. with minimal to no wheelspin). AWD cars and rear-engine cars usually have the highest Launch values. Launch ratings are normally not shown in Forza Motorsport 6 and Forza Motorsport 7.
- Braking: How well the car can brake to a stop. This is determined by distances required for 60 mph (97 km/h) - 0 and 100 mph (161 km/h) - 0.
It is shown by letters (F to A, then S, S1, S2, R, P, U, or X) and then a number starting from 100 and capping at either 999 or 1000 (e.g. P 998).
Any vehicle's performance index can be increased by applying power-improving and weight-saving upgrades. Players can also decrease their P.I. value by adding weight and using a drivetrain conversion.
In the Forza Horizon games, there are no cars ranked X 999 by default.