Great idea, the quickest and cheapest way to adapt racing image to your car.
Spend a little extra time in preparation - to find both the right size and position. If wrong it just looks strange and unbalanced.
I buy the vinyl (or whatever it is) in a roll from a sign maker and use a compass with a scalpell blade attached to it. Matt vinyl looks better than glossy. Use a "flower sprayer and a rubber scraper" when positioning.
On the sides: if positioned slightly closer to the front wheel than rear the car looks faster (depending on the design of the car). On the bonnet/rear: do not place it directly in the middle, offset positioning looks better (think racing track, very few spectators watching from above car). Very double curvatured surfaces are tricky to cover, the material is possible to stretch a little or just cut it (from he center and out).
Clean the surface before applying the roundel to make sure no dirt lifts the vinyl, wax the car before. Afterwards be careful with water pressure directly to the edges when washing. If leaving it on for ages...it might be hard to remove in one piece - sunlight and age makes it brittle with time.
More than three roundells/car looks often to much...
If you also are going for a number, be careful to find a font, size and number to match your car.
I once had a question from a girl if the white spots on my car was to make it look like a ladybird...
Happy racing!
