Ford F-550 Super Duty Dome Light Bulb Size

The Ford F-550 Super Duty dome light utilizes 578 and 912 bulb sizes for interior illumination across various production versions. Data for the specified lighting components covers the range from 1999 through 2016.

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
2005-2016	578
2003-2004	912
2002	578
1999-2001	912

Bulb Specifications

Description	Bulb Size
ANSI standard (US & Canada). Festoon bulb, 41mm length, end-cap contacts. 12V, 12W.	578
ANSI standard (US & Canada). Wedge bulb (T15). Variant of 921. 12V, 12W.	912

Other Locations

Additional Notes

The Ford F-550 Super Duty dome light uses one of two bulb types across its model years: the 578 festoon bulb or the 912 wedge bulb, both rated at 12V and 12W.
The 578 is a festoon-style bulb measuring 41mm in length with end-cap contacts, meaning it seats by pressing its two flat metal ends into spring clips rather than inserting into a socket.
The 912 is a T15 wedge bulb, a variant of the more widely known 921, and connects by pressing its base directly into a wedge-style socket without requiring a separate contact mechanism.
Both the 578 and 912 carry ANSI standardization for the US and Canadian markets, indicating they conform to established electrical and physical tolerances for those regions.
The Ford F-550 Super Duty uses the 578 festoon bulb for model years 2005-2016, representing the longer continuous span within the listed specifications.
The 912 wedge bulb is specified for the 1999-2001 and 2003-2004 model years, while the 578 festoon bulb is specified for 2002, making the two types non-interchangeable due to their fundamentally different socket and contact designs.
Because the 578 uses end-cap contacts and the 912 uses a wedge base, the two bulb types require physically different socket housings and are not cross-compatible with each other.
Although both bulbs share the same 12V, 12W electrical rating, their differing physical formats mean wattage equivalence alone does not make one a substitute for the other.
The 912 is classified as a variant of the 921 bulb, which shares the same T15 wedge base but differs in filament or output characteristics, so a 921 is not necessarily a confirmed replacement for the 912 in this application.