Minecraft wiki entry for Mule, associated text: A ridden mule automatically runs up any one block high slope. The mule and rider can safely fit through a space as low as 2.75 blocks high. Lower clearance risks suffocating the rider if the rider's head enters a non-transparent block. The mule itself can enter gaps as low as 1.625 blocks high, but may itself take suffocation damage when clearance is less than 1.75 blocks. Mules cannot fit through a 1-block-wide gap.