Ford E-350 Super Duty Dome Light Bulb Size

Additional Notes

• The Ford E-350 Super Duty dome light draws from three distinct bulb types across its model years: the 578, the 912, and the 6411.
• The 578 and 912 are both rated at 12V and 12W, making them electrically equivalent, though their physical forms differ significantly.
• The 578 is a festoon-style bulb measuring 41mm in length with end-cap contacts, meaning it seats by pressing lengthwise between two spring clips rather than inserting into a socket.
• The 912 is a wedge-style bulb in the T15 form factor, which uses a plastic base with wire contacts that grip when inserted into a wedge socket.
• Because the 578 and 912 use fundamentally different mounting systems, one cannot substitute for the other without a compatible housing.
• The 912 is a variant of the 921 bulb, sharing the same wedge base and voltage rating, though the 921 operates at a higher wattage of 18W, making the two non-interchangeable as direct replacements.
• The 6411 is also a festoon bulb but is shorter at 36mm and uses an SV8.5 base, distinguishing it physically from the 578 despite both being festoon types.
• The 6411 is rated at only 5W, considerably lower than the 12W output of the 578, so the two festoon bulbs are not interchangeable despite sharing a similar form factor.
• The Ford E-350 Super Duty (2021-2023) uses the 578 bulb for the dome light, while the 2019 model year also shares this fitment.
• The 6411 specification is isolated to the 2017 model year, with adjacent years specifying different bulb types entirely.