Only thing I can find in the Texas Vehicle code regarding Fenders/Mud Flaps is this:
Sec. 547.606. SAFETY GUARDS OR FLAPS REQUIRED. (a) A road tractor, truck, trailer, truck-tractor in combination with a semitrailer, or semitrailer in combination with a towing vehicle that has at least four tires or at least two super single tires on the rearmost axle of the vehicle or the rearmost vehicle in the combination shall be equipped with safety guards or flaps that:
(1) are of a type prescribed by the department; and
(2) are located and suspended behind the rearmost wheels of the vehicle or the rearmost vehicle in the combination within eight inches of the surface of the highway.
(b) This section does not apply to a truck-tractor operated alone or a pole trailer.
(c) In this section, "super single tire" means a wide-base, single tire that may be used in place of two standard tires on the same axle.
Also, see here:
In Oregon My wheels aren't supposed to go past the Fender flares, but I got about 2-3 inches past the flare and haven't been stopped. I've had 3-5 cops see me in the last month but never pulled me over (including one directly behind me). I'm sure I'm just playing with fire until I get that one City Cop who didn't get any the night before and cite me for it and when he does I'll just order 6" fender Flares and solve that problem.