The GMC P60 front and rear turn signal bulbs utilize either 1157 or 2057 size specifications. The replacement chart covers 1983 to 1990 production 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 1157 and 2057 are the two bulb types specified for turn signal use on the GMC P60, and both conform to ANSI standards recognized in the United States and Canada.
Both bulb types share the BAY15d bayonet base, a dual-contact base design that physically accommodates two filaments within a single housing.
Both the 1157 and 2057 operate on a 12-volt electrical system, which is standard for automotive applications of this era.
The dual-filament design of both types serves two distinct functions within a single bulb, with the higher-wattage filament handling the turn signal and the lower-wattage filament serving as a running or parking light.
The 2057 draws 26.9W on its primary filament and 8W on its secondary, compared to the 1157 at 21W and 5W, making the 2057 a higher-output variant despite sharing the same base type.
The GMC P60 front turn signal used the 2057 bulb for the 1983-1984 model years, then transitioned to the 1157 for the 1985-1990 model years.
Although the 1157 and 2057 share the same BAY15d base, their differing wattage ratings mean they are not identical in output, and substituting one for the other affects light intensity.
The rear turn signal on the GMC P60 used the 2057 exclusively in 1983, while the 1157 was specified for all rear turn signal positions from 1984 through 1990.
In 1984, the GMC P60 used different bulb types for the front and rear turn signals simultaneously, with the 2057 at the front and the 1157 at the rear.