Buick Gran Sport Back Up Light Bulb Size

The Buick Gran Sport back up light requires bulb size 1156 or P21W depending on the specific application for the vehicle. Data for the bulb sizes covers the range from 1965 to 1967.

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
1965-1967	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 sole bulb type listed for the back up light across all documented model years of the Buick Gran Sport, spanning 1965-1967.
The 1156 is an ANSI-standardized bulb, meaning it conforms to a defined specification recognized across the United States and Canada, ensuring consistent performance and physical dimensions from one manufacturer to another.
It operates on a 12-volt electrical system at 21 watts, which is consistent with the standard automotive electrical architecture of that era.
The 1156 uses a BA15s bayonet base, a single-contact mount where the "s" denotes a single pin. This distinguishes it from the visually similar BA15d base, which carries two pins and is not interchangeable.
As a single-filament bulb, the 1156 serves one dedicated function per socket. This contrasts with dual-filament types such as the 1157, which handle two functions from a single housing. The two types share a similar physical form but are not interchangeable due to base pin differences.