There are five non-BMW classes, X1, X2, X3, X4 and X5. Class X5 is reserved for all non-BMWs with R-Compound tires (defined as having a treadwear rating less than 200). The remaining classes are based on vehicle displacement and modified according to the rules below.
X1 = 5.0 Liters and up (Non-BMW 1)
X2 = 3.7 – 4.9 Liters (Non-BMW 2)
X3 = 2.6 – 3.6 Liters (Non-BMW 3)
X4 = 2.5 Liters or less (Non-BMW 4)
X5 = R-Compound Tire Cars Only (no matter engine size) (Non-BMW 5)
After the base group is determined from the above table, move up one group as follows:
-Increase one group if engine uses forced induction (either factory or aftermarket).
-Increase one group if engine is rotary.
X5 is specifically reserved for cars with competition tires no matter what engine the car is equipped with.
-A 2018 Mazda Miata (2.0) would be in X4. However, a 2018 Fiat 124 Spyder (1.4) which is essentially the same car would be in X3 due to the turbocharger.
-A 2010 Subaru WRX STI (2.5L) moves up from X4 to X3 because it has forced induction. Even factory forced induction, such as in this case, causes a car to move up 1 Group.
-A 2010 Chevy Camaro V6 (3.6) would be in X3. A V8 (6.2) would be in X1 no matter the trim (LT, LS, SS, ZL1, etc.). The same car with R-Compound tires would complete in X5 no matter the engine size.
-A 2008 Mazda RX-8 (1.3L) would be in X4, but is an X3 car because of its rotary engine. If the same RX-8 competes with R-Compound tires, it is in X5.