The Mack RD front and rear side marker bulb size is 1891 for this truck model. The replacement chart covers vehicle production years from 1981 to 1997.
The year-by-year compatibility chart below provides a quick reference to verify the correct bulb size for your vehicle.
The Mack RD uses the 1891 bulb in both the front and rear side marker positions across all listed model years, 1981-1997.
The 1891 is an ANSI-standardized bulb recognized in both the United States and Canada, meaning it conforms to a defined set of electrical and physical specifications shared across those markets.
The 1891 belongs to the T5 bulb family, a miniature wedge type. The T5 designation refers to a tubular glass envelope that is 5/8 of an inch wide, and the wedge base replaces a traditional metal socket pin with a pinched glass base that wedges directly into the housing.
The 1891 operates at 12 volts, which is consistent with standard 12-volt automotive electrical systems.
At 2 watts, the 1891 draws very low current, a characteristic typical of side marker bulbs, which are indicator lamps rather than primary illumination sources.
Because the front and rear side marker positions on the Mack RD share the same bulb specification, a single bulb type services both locations on the vehicle.
The wedge base of the 1891 is not interchangeable with bayonet-base or screw-base bulb types, even when wattage or voltage ratings are otherwise similar.