For the most part the new Group 5/Imsa cars in S and R class are pretty accurate as far as performance goes in relation to lap times when the cars raced.However there are some P cars that are just not accurate.The new Ganassi Ford should be at least level with the Mazda Lola.The championship winning Toyota is junk compared to the Audi even if you do modify it.
The glaring error however which has been repeated since Forza 4? The ridiculous performance of the Mazda 787b and the lack of performance of the Peugeot 905C.
Mazda Handling rating 8.8
Peugeot Handling rating 8.7
1991 Mazda lap time at Suzuka 1.55.8
1992 Peugeot lap time at Suzuka 1.45.0
1991 Mazda lap time at Le Mans 3.43.5
1993 Peugeot lap time at Le mans 3.24.9
I have no problem with the Mazda being modified to 998 in the P class but it should only be competitive at tracks like Le Mans and low downforce tracks.This car was designed to win at Le Mans and should not be anywhere near the newer cars on downforce tracks.It simply does not have the downforce that the newer cars or the Peugeot 905 and there is no way to make it have the downforce other than a giant rear wing which would kill the straight line speed.
The Mazda was a modern version of the Porsche 962/956 and the Peuegeot was an F1 with a roof.
The Peugeot 905C should have a PI of close to 998.
The Mazda787b should have a PI of close to 950
I know Forza is not a simulation but when the developers talk about tire physics etc then they should portray racing cars as they are and not something else.