GMC R2500 Headlight Bulb Size

Additional Notes

• The GMC R2500 headlight specifications cover three distinct headlamp system configurations across the 1987-1989 model years: a 2-headlamp rectangular system, a 4-headlamp rectangular system, and a halogen capsule system. Each configuration requires a different set of bulb sizes for high and low beam functions, meaning the correct bulb size is determined by which headlamp system is installed on the vehicle.
• The H6054 is a 5x7-inch rectangular sealed beam unit operating at 12V and conforming to ANSI standards for the US and Canada. It serves as both the high beam and low beam in the 2-rectangular-headlamp configuration across the 1988-1989 model years, functioning as a dual-purpose unit rather than separate high and low beam components.
• Sealed beam units, including the H6054, H4701, H4703, H4651, and H4656, are not replaceable bulbs in the conventional sense. The entire sealed unit must be replaced when the light source fails, as the filament, reflector, and lens are fused into a single assembly.
• The 4-headlamp rectangular configuration uses different sealed beam units for high and low beam functions. In 1988-1989, the H4701 handles high beam and the H4703 handles low beam. In 1987, the H4651 handles high beam and the H4656 handles low beam. The H4701 is described as a variant of the 7-inch round sealed beam format, while the H4703 is the standard 7-inch round unit.
• The 9005 and 9006 are single-filament halogen bulbs used in the halogen capsule headlamp configuration available on the 1988-1989 GMC R2500. The 9005 is rated at 65W with a P20d base for high beam duty, while the 9006 is rated at 55W with a P22d base for low beam duty. Despite their similar appearance, the P20d and P22d bases are not interchangeable, as the physical key positions differ to prevent incorrect installation.
• The 10W power difference between the 9005 high beam and the 9006 low beam reflects a standard engineering practice in halogen headlamp design, where high beam filaments are driven at higher wattage to produce greater light output for long-range visibility.
• The 4x6-inch rectangular sealed beam units used in the 1987 4-headlamp configuration, H4651 for high beam and H4656 for low beam, are distinct part numbers despite sharing the same physical format. Substituting one for the other would result in incorrect beam pattern placement within the headlamp system.
• All bulb types listed across the GMC R2500 specifications conform to ANSI standards applicable in both the US and Canada, and all operate on a 12V electrical system.