Mud flap and Splash-guard

Splash-guard and mud flap are semantically related In some cases you can replace phrase "Splash-guard" with "Mud flap", this phrases are similar.

Splash-guard noun - A curved piece above the wheel of a bicycle or motorcycle to protect the rider from water or mud thrown up by the wheels.

Mud flap and splash-guard are semantically related in flap for keeping dirt off vehicle topic. You can use "Mud flap" instead a noun phrase "Splash-guard".