Variable speed V-belts are specialized V-belts designed for applications where the speed of the driven machine needs to be adjusted or changed frequently. They are part of a wider family of V-belts but have a distinctively broader width and narrower thickness, allowing for smoother speed changes and excellent acceleration, according to Acorn Industrial Services and Mitsuboshi. These belts are particularly useful in applications where speed adjustments are common, such as in variable speed pulley systems.