Okla. Stat. tit. 47, § 47-12-405.3: Aprons

All vehicles or combination of vehicles operating on the

highways, except animal-drawn vehicles, not equipped with fenders

over the rearmost wheels shall have attached thereto a rubber or

fabric apron directly behind the rearmost wheels, and hanging

perpendicular from the body of the vehicle. The apron shall be of

such a size as to prevent the bulk of the water or any other

substance picked up from the roadway from being thrown from the rear

wheels of the vehicle or combination of vehicles at tangents

exceeding twenty-two and one half (22 1/2) degrees measured from the

road surface. The provisions of this subsection shall not apply to

a farm tractor moving over the state highway system at a speed less

than twenty (20) miles per hour.