The Mack RS front and rear side marker bulb size is 1891, with both positions utilizing the same bulb size. The replacement chart contains vehicle production years and bulb size specifications spanning from 1981 to 1989.
The year-by-year compatibility chart below provides a quick reference to verify the correct bulb size for your vehicle.
The Mack RS uses the 1891 bulb for both its front and rear side marker positions across the 1981-1989 model years.
The 1891 is classified as a T5 miniature wedge bulb, a designation that refers to its tubular glass envelope measuring 5/8 of an inch in width.
The wedge base design means the 1891 bulb is held in place by friction from the socket itself, with no separate metal base or locking pins involved.
The 1891 operates at 12 volts and draws 2 watts, placing it among the lower-output bulbs used in commercial vehicle marker lighting.
The 1891 conforms to ANSI standards recognized in both the United States and Canada, indicating a standardized specification for its electrical and physical characteristics.
Because the same bulb number applies to both the front and rear side marker positions, a single bulb type services all side marker sockets on the RS throughout the listed model range.
The T5 form factor is a compact one, commonly associated with indicator and marker applications rather than high-output lighting functions such as headlamps or driving lights.
The low wattage of the 1891 reflects its role as a visibility and position marker rather than an illumination source, consistent with regulatory requirements for side marker lamps on commercial vehicles.