I have noticed that the performance index cost for parts are different from car to car.
In fact it is different based on parts already added to the car.
Is there anyway to tame this horse and get accurate PI costs for upgrades across cars without having to brute force every permutation?
Is consistency too much to ask 
brouhaha
Iām not sure I understand your question. Are you talking about auto-upgrades or the price of the air filter varying from car to car?
air filter from car to car, or any part for that matter
It wouldnāt make any sense to have one PI ācostā for each part that stays the same from car to car. Adding an air intake that adds 10 HP to a 1,500 lb lotus(just making numbers up here) that only had one hundred something HP to start with is completely different from putting an air intake on a 5,000 lb viper and adding 10 HP to a car that already had 500 HP. One would make a much bigger difference in performance than the other. The PI should reflect how much the upgrade affect performance on that carā¦even if it doesnāt do it as well as I would like.
Take car to car out of the discussion. Why does the PI change for the same part after Iāve added other parts to the same car?
Kitty - u r correct, and, this has actually bugged me for awhile, yet, I guess thatās how the game is structured. But, to your point, the sequence of adding the parts is at times odd. That is why nowadays I always go back after my final critical build selections to ensure if my sequence is different can I gain some decrease in weight for example, and so forth.
Sometimes it can also be aero, gearing or something else you have added that allows the car to handle more speed.
Fixed ratio, rev banging gearboxes can hold back the top speed of cars. Adding hp will not significantly increase top speed. Change the gearbox and adding hp results in greater performance gains and therefore may have different pi costs depending what you have done before.
It is also the case that sometimes weird things happen though so sometimes the different pi may make no sense even when taking account of these other factors but other times there is an explanation.
Another example is roll cages. Put one on a D class car with stock motor and you wonāt see many green stats and PI may not shift much. Max out the motor before adding the roll cage and all of a sudden the roll cage is more useful and often has an ever increasing but still small pi cost.
Thanks! Glad to know Iām not going crazy.
Becuase having a good balace of speed and grip costs more pi.
If I have a powerful/low handing car, and I keep adding power the power cost less than normal and if I keep adding more and more power is will cost less and less PI.
If I added power to a grip car thats max out its grip it will go up a lot of PI.
Makes sense (logical)!