The Mack RB front and rear side marker bulb size is 1891, a specification applicable to the vehicle model across its production range. The replacement chart covers 1989 to 1997 model years with corresponding bulb size data.
The year-by-year compatibility chart below provides a quick reference to verify the correct bulb size for your vehicle.
The 1891 bulb is the sole bulb type specified for side marker applications on the Mack RB across all listed model years.
The Mack RB uses the 1891 bulb for both front and rear side marker positions across the 1989-1997 model years.
The 1891 is classified as a miniature wedge bulb, meaning it uses a wedge-style base rather than a bayonet or screw base, and seats directly into a wedge socket without a separate metal cap.
The T5 designation refers to the bulb's tubular envelope, which is 5/8 of an inch wide, making it among the smaller bulb forms used in automotive applications.
The 1891 operates at 12 volts and draws 2 watts, placing it at the low end of the power range for automotive marker lighting.
The 1891 conforms to ANSI standards recognized in both the United States and Canada, indicating it meets a standardized set of electrical and physical specifications in both markets.
Because the same bulb number applies to both the front and rear side marker positions, a single bulb type serves all side marker sockets on the vehicle.
The wedge base format of the 1891 is not interchangeable with bayonet-base bulbs of similar wattage, as the socket type differs fundamentally in its retention and contact method.