Took a while, but got the smoke sorted for all 4 wheels and grit for the rear left wheel (you can see grit in the above rendered images). It really does add another dimension to the film and make things look much more real. Below is a video giving you an idea how the smoke is done. So for smoke is finished but the grit needs to be applied to the other wheels. To help with CPU time, the particles disappear after a given period and even then it’s randomised so it doesn’t look obvious when they particles start disappearing.