Not sure if this helps, but we do a TON of magnetic letter/numbers for use on autocross and track cars. We use .030 magnum for everything. Assuming the surface is smooth and there is not a ton of bondo on the car, it will generally stay put up to ~70mph. People who use it on track at higher speeds have more issues and we generally recommend other products for them.
No matter the speed, if it is not put down flat on the surface and has any air gap, it will blow off. We have tested with .060 material and it does stick better but the profile is so high that it still blows off at speed anyway. Maybe not an issue for your application.
DaveW