Choose wheels based on riding style, and also for looks! Wheel sizes are in millimeters (mm) for their diameter. Smaller wheels at 48mm-54mm are slower and typically for street/technical riding. Wheels between 55mm to 60mm are best for skate parks, ramps, or vert due to their faster/smoother ride. Wheels over 60mm are for cruiser boards and longboards. Another important feature in wheels is their hardness indicated by the "Durometer scale". Hard wheels (96a to 100a) are fastest with less grip, soft wheels (75a-87a) are good for rough surfaces and have lots of grip (as needed for longboards).