 The new AI wheels folder: \base\vehicle\wheel\car\generic.
In this folder will be some generic wheels that are used accross many diferent cars. The tires and wheels sizes there should cover most of the commonly used wheels.
Because the real world tire and rim sizes are too numerous and complex for the purpose of the game things should be simplified to a several different categiries.
Something like normal, low profile and off-road ( higher profile tires suitable for SUV or light passenger trucks ) plus the wheel size in inches.

16in_normal      = P205/60 R16 
17in_normal      = P215/55 R17
17in_offroad     = P255/75 R17
18in_normal      = P245/50 R18
19in_lowprofile  = P245/40 R19
20in_normal      = P245/45 R20
20in_offroad     = P265/60 R20
24in_offroad     = P285/40 R24 


This categories are more or less approximate and should not be mandatory in any way but just to let you know that the side profile of the rubber part of the tire is higher, lower or standard.
The proper wheels should be those that look most convinsing and proportionaly correct in the engine. Which unfortunately is not always the manufacturer recomended tire and wheel size because sometimes the proper size looks a miniature in game next to the car body and in that case is better to pick a different size until it looks accurate.
It maight be because of error due to use of bad blueprint or else, but after all if it looks right noone willl measure the cars proportions down to a centimeter inside the game. Hoever if the tires look wrong - miniature, too tick or too large everyone will be able to notice it even if they are not an engeneares or car designers. And poorly scaled tires can make even the best model to look weak, unstable or ugly.

If for some reason you need to make a different size of a wheel that fits better to your model ... for the sake of keeping things organised the custum wheels which are going to be created to be used with a specific car should contain the name of the vehicle for which they will belond ( like a Ford,Toyota... ), or the model, or type ( like a police car, post office car and so on... ) plus the wheel size in inches and should be placed in folder
\base\vehicle\wheel\car.

Source wheels textures: \samba\staff\vladi\Wheels textures\

Good websites for referencing a passenger car tires and wheels sizes:

http://www.tirerack.com/content/tirerack/desktop/en/homepage.html

http://www.wheel-size.com/calc/?wheel1=295-55-20X10.5ET45&wheel2=225-45-17X7.5ET40&fcl=50mm&wcl=30mm&scl=50mm&sr=0mm