Highway Wheels is an addon in Spintires and MudRunner. Although they still offers their advantages and trade-offs of a typical addon, in MudRunner they are considered "stock" or "permanent" since the ability to change wheels has been removed.
Highway Wheels are standard truck wheels designed for use on paved roads. They consist of single wheels on the front axle, and dual wheels on the rear axle(s), and narrower, lower-profile highway-terrain or on/off-road tires are used instead of larger, wider, much more aggressive mud-terrain tires of the Big Wheels. Their rims are also those of standard on-highway trucks instead of the heavier duty off-the-road and usually CTIS-equipped rims of their counterparts.
While they considerably increase the fuel economy of the trucks they are mounted on, they do not negotiate well with difficult terrain, since they offer much lower grip in poor terrain. As per their namesake, they should only be used on paved roads or other hard surfaces for light offroading only. Going into swamps, deep mud, or rivers with these tires is obviously highly discouraged.
In MudRunner, the addon is used by the B-130, C-65115 and C-256 in the base game, as well as the Ford LTL9000, Chevrolet Bison, and Freightliner FLD120 in the American Wilds DLC, and the GMC DW950 in the Old Timers DLC. Due to MudRunner no longer offers the player the ability to change wheels, they are permanently installed on said vehicles and cannot be swapped out.
They are also used on all trailers sans semi-trailers (except American Wilds' semi-trailers), and log carts (again, except American Wilds' Log Carts, where the wheels of the Western Star 6900XD are used for the semi-trucks' Long Log Cart; or the wheels of the Ford F-150 are used in the case of the Loaded Log Cart, exclusive in the Rocky Hills map of the Old Timers DLC). Obviously, since the trailers' wheels are not powered, they make little to no difference compared to a more aggressive tires and wheels setup, as the performance is entirely dependant on that of the vehicle towing the trailer.