GMC G25 Back Up Light Bulb Size

The GMC G25 back up light uses a 1156 bulb size for the reverse illumination function. The replacement chart covers vehicle production spanning 1973 to 1978 with corresponding bulb specifications.

The year-by-year compatibility chart below provides a quick reference to verify the correct bulb size for your vehicle.

Bulb Size by Year Range

Years	Bulb Size
1973-1978	1156

Bulb Specifications

Description	Bulb Size
ANSI standard (US & Canada). Single-filament bulb with BA15s bayonet base. 12V, 21W.	1156

Other Locations

Additional Notes

The 1156 is the back up light bulb size for the GMC G25 across the 1973-1978 model years.
The 1156 bulb follows the ANSI standard, meaning it conforms to specifications recognized across the United States and Canada.
It is a single-filament bulb, which means it serves one lighting function only, as opposed to dual-filament bulbs that handle two functions such as braking and running lights simultaneously.
The 1156 uses a BA15s bayonet base, a single-contact mount where the bulb is inserted and twisted to lock into place.
The bulb operates at 12 volts, consistent with the standard electrical system voltage found in most conventional automotive applications of that era.
At 21 watts, the 1156 produces a relatively high output for a single-filament bulb, which is typical for back up light applications that require adequate illumination behind the vehicle.
The BA15s designation indicates a single-contact bayonet cap with a 15mm base width, and substituting a BA15d or other multi-contact base bulb would result in an incompatible fit.