Well my Kit lens is 52mm (18 - 55), But I dont use that for Motorsport, My 70-300mm AF-S is 67mm Diameter, so will just buy a filter for that Lens...
Polarising filters are great and I wouldn't be without one.
Great for reducing glare from glass, car paintwork, shiny roads, vegetation etc. Also darkens sky and brings the blue out to get clouds looking great.
Best at 90 degrees to the sun, i.e not photographing towards or away from the sun.
Here's the bad bit, perspex and transparent plastic do not give good results unless you are a big fan of rainbows.
Cars often have perspex or plastic headlight lenses, some might have perspex or similar windshields.
A polariser might not be right for your motorsport photos, you really need to have one in your hands and play with it to see how it works with different materials.