An all-terrain vehicle is supported and driven by endless track units. Each track unit includes a drive belt with rows of interior drive lugs driven by cogs on multiple sprocket wheels of a common drive wheel assembly having an integrated, one-piece drum and hub. Multiple guide wheel assemblies engaging the lower run of the drive belt distribute the vehicle load across substantially the full width of the belt. Each guide wheel assembly includes a pair of guide wheel units mounted on opposite end portions of a common spindle and on opposite sides of a carrier beam of the track unit. Each guide wheel unit has a wide belt-engaging surface extending to, or close to, an edge portion of the belt and interrupted by an annular groove permitting passage of a row of the belt drive lugs while maintaining alignment of the belt and drive wheel assembly.