GMC W7500 Forward Back Up Light Bulb Size

Additional Notes

• The two bulb types specified for back up lights across the GMC W7500 Forward model years are the 1073 and the 1156, both of which are ANSI-standardized single-filament bulbs rated at 12 volts and sharing the BA15s bayonet base.
• The BA15s bayonet base is a single-contact design, which is a defining physical characteristic of both the 1073 and 1156 bulbs listed in these specifications.
• The primary electrical difference between the two bulb types is wattage. The 1156 operates at 21W, while the 1073 operates at 13W, making the 1073 a lower-output variant of the same base family.
• Because the 1073 and 1156 share the same BA15s bayonet base, they are physically interchangeable in terms of socket fitment, though their wattage ratings differ and the specifications assign each to distinct trim configurations.
• The Tiltmaster configuration of the GMC W7500 Forward is specified with the 1073 bulb across multiple model years, including 1984, 1988-1989, and 1993-1996.
• The Medium Duty configuration is specified with the 1156 bulb for the 1984, 1988, and 1989 model years.
• For 1985 and 1986, both the 1073 and 1156 are listed without a named sub-configuration, indicating that two separate fitments existed within those model years.
• The 1156 bulb is the sole specified fitment for the 1987, 1990-1992 model years, with no secondary bulb listed for those years.
• The 1994 model year is the only year within the Tiltmaster entries where the 1156 replaces the otherwise consistent 1073 specification, representing a deviation from the pattern seen in surrounding Tiltmaster years.
• Both the 1073 and 1156 are recognized under ANSI standards in the United States and Canada, confirming their use within North American automotive lighting systems.