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