Increase, keep or descrease car classes on Forza Horizon 2?

I'm playing FH2 since yesterday and I loved it. I'm trying to learn the game mechanics, builds, setups, tuning, etc. Should I always upgrade my cars to higher classes? When should I increase, keep or decrease my car level? Which are the pros and cons?

Thanks!

xbox-one forza-horizon-2

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What do the car classes in Forza mean?

Forza Support Team. Classes, Performance Index and Ratings. Every Car has a Performance Index (PI) that indicates the overall performance of the vehicle. PI is a number from 0-999, the higher the number the faster the car. PI determines the class of the car, from D, C, B, A to S1, S2, and X class.

What does class mean in Forza?

Class evaluates overall performance determined by a car's lap time on a simulated test track.

How fast is the fastest car in Forza Horizon 2?

Depends what you mean by \u201cfastest\u201d. Several cars top out around 270-275 mph, but the Alfa Romeo 33 Stradale, converted to AWD, will reach that speed fastest.

Forza Horizon 2 - Fastest way to earn skill points (wallgrind method)

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Images: Nubia Navarro (nubikini), Pixabay, Mario Cuadros, Maria Orlova