Cadillac Fleetwood Headlight Bulb Size

Additional Notes

• The Cadillac Fleetwood headlight specifications span three distinct bulb technologies across the model's history. The 1965-1974 model years used sealed beam units in round and square form factors, the 1975-1986 model years transitioned to rectangular sealed beam units, and the 1987-1996 model years adopted replaceable halogen bulb technology with separate high and low beam housings.
• The 9005 and 9006 halogen bulbs, fitted to the 1987-1996 Cadillac Fleetwood, are single-filament halogen types operating at 12V. The 9005 is rated at 65W for high beam duty, while the 9006 is rated at 55W for low beam duty. Each uses a distinct base type, with the 9005 using a P20d base and the 9006 using a P22d base, making the two bulbs physically non-interchangeable despite their similar appearance.
• The H4651, H4652, and H4656 bulbs used across the 1975-1986 model years are all 4x6-inch rectangular sealed beam units operating at 12V. These are complete self-contained assemblies in which the lens, reflector, and filament form a single sealed unit. Replacement requires substituting the entire assembly rather than an internal bulb component.
• The H4652 is specified as a variant of the H4651. Both are 4x6-inch rectangular sealed beam units, though they serve the low beam and high beam positions respectively. The H4656 also occupies the 4x6-inch rectangular form factor and is used in the low beam position across several model years within the 1978-1985 range.
• The low beam specification for the Fleetwood between 1975 and 1986 is not uniform. The H4652 is listed for 1975-1977, 1982-1983, and 1986, while the H4656 is listed for 1978-1981, 1984-1985. These two sealed beam types are not confirmed interchangeable by the specifications, and substituting one for the other without verification of fitment compatibility carries risk.
• The H5001 and H5006 sealed beam units, used in the 1965-1974 model years, differ in form factor from the later rectangular units. The H5001 is a 4-inch square sealed beam unit for high beam, and the H5006 is a 5.75-inch round sealed beam unit for low beam. Both operate at 12V and are complete assemblies requiring full unit replacement.
• All bulb and sealed beam types listed across the full specification range conform to ANSI standards applicable in the United States and Canada. This standardization means that compliant replacement units from any manufacturer meeting the relevant ANSI designation are dimensionally and electrically equivalent to the original equipment specification.
• The transition from sealed beam technology to replaceable halogen bulbs between the 1986 and 1987 model years represents a fundamental change in headlight serviceability for the Fleetwood. Sealed beam units from the earlier years require full assembly replacement upon failure, whereas the 9005 and 9006 halogen bulbs used from 1987 onward allow the bulb alone to be replaced within a retained housing.