The GMC P4500 front and rear turn signal bulbs use either 1157 or 2057 size specifications. The replacement chart covers 1979 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 GMC P4500 uses two bulb types across its front and rear turn signal positions: the 1157 and the 2057, both of which are ANSI-standard dual-filament bulbs rated at 12 volts and fitted with a BAY15d bayonet base.
The 1157 bulb operates at 21W on its high-filament circuit and 5W on its low-filament circuit, making it the standard dual-filament choice for automotive turn signal and parking light applications.
The 2057 bulb operates at 26.9W on its high-filament circuit and 8W on its low-filament circuit, placing it at a higher wattage output than the 1157 across both filaments.
Both the 1157 and 2057 share the BAY15d bayonet base, which is a dual-contact offset-pin base. The offset pin positions prevent incorrect insertion orientation.
Despite sharing the same base type, the 1157 and 2057 are not identical in output, and substituting one for the other may result in a turn signal circuit that does not meet the original wattage specification for a given model year.
The front turn signal position on the GMC P4500 uses the 1157 for model years 1979-1982 and 1984-1990, while the 2057 appears exclusively in the 1983 model year for that same position.
The rear turn signal position uses the 2057 for the 1983-1984 model years, while the 1157 is specified for 1979-1982 and 1985-1990.
The 1983 GMC P4500 is the only model year in these specifications where both the front and rear turn signal positions share the same bulb type, with the 2057 fitted at both locations.
All entries across both turn signal positions carry a Medium Duty classification, which reflects the vehicle's commercial truck designation rather than a characteristic of the bulb itself.
The 1157 is the predominant bulb type across the full span of these specifications, appearing in the majority of front and rear turn signal positions throughout the 1979-1990 range.