Adjusting tire smoke particle emitter based on wheel slip

Hello ! I have made a car using WheelCollider's and added a particle emitter for tire smoke on each wheel. Problem is, the emitters are constantly emitting a constant amount of smoke. I would like them to emit smoke proportionally to how much the associated wheel is slipping, i.e the different between the speed of the tire surface, and the road in contact with it.

Do I have to manually compute that "slipping" manually or is Unity already providing something similar ? The WheelCollider class doesn't seem to contain the info I would like (other than RPM, radius, etc).

Thanks for your help !