Ford F-350 Super Duty Dome Light Bulb Size

Additional Notes

• The Ford F-350 Super Duty dome light uses two distinct bulb types across its model years: the 578 festoon bulb for 2005-2016 and the 912 wedge bulb for 1999-2004.
• The 578 is a festoon-style bulb measuring 41mm in length with end-cap contacts, meaning the bulb makes electrical contact at each end rather than through a wedge base.
• The 912 is a T15 wedge-base bulb, a form factor in which the bulb's glass envelope itself acts as the base, inserting directly into a wedge socket without a separate cap or pin.
• Both the 578 and 912 operate at 12 volts and 12 watts, placing them in the same electrical load category despite their different physical bases.
• The 912 is classified as a variant of the 921 bulb. While the two share the same T15 wedge base and voltage rating, they are not identical, and substituting one for the other without confirming socket and wattage compatibility is inadvisable.
• Both bulb types carry ANSI standardization for the US and Canadian markets, meaning their electrical and physical specifications conform to a defined industry standard rather than being proprietary to Ford.
• The festoon base of the 578 and the wedge base of the 912 are physically incompatible socket types, so the two bulbs are not interchangeable with each other regardless of their matching voltage and wattage ratings.
• The 2004 Ford F-350 Super Duty marks the transition point between the 912 and 578 bulb types, with 2004 and earlier model years using the 912 and 2005 and later model years using the 578.