GMC C25 front and rear turn signal bulbs utilize sizes 198, 199, 1073 for front applications and 198, 199, 1156 for rear applications. The replacement chart covers 1967 through 1978 vehicle production years with corresponding bulb size specifications.
The year-by-year compatibility chart below provides a quick reference to verify the correct bulb size for your vehicle.
The GMC C25 front turn signal uses three distinct bulb types across its 1967-1978 model range: the 1073 for 1967-1968, the 199 for 1969-1972, and the 198 for 1973-1978.
The bulb 198 is an ANSI-standard miniature wedge type in a T5 form factor, rated at 12V and 2W, making it significantly lower in wattage than the other bulb types specified for this vehicle.
The bulb 199 is an ANSI-standard single-filament type with a BA15s bayonet base, rated at 12V and 21W, and is described as a variant of the 1156.
The bulb 1073 shares the same BA15s bayonet base as the 199 and 1156 but is rated at 13W rather than 21W, making it a lower-wattage variant within the same base family.
The bulb 1156 is an ANSI-standard single-filament type with a BA15s bayonet base, rated at 12V and 21W, and serves as the reference bulb for the 199 and 1073 variants.
Because the 199 and 1156 share the same BA15s base and identical 12V/21W ratings, they are electrically and physically comparable, though they are catalogued as separate ANSI designations.
The 1073, while fitting the same BA15s base as the 1156 and 199, draws only 13W versus their 21W, which represents a meaningful difference in output and should be considered when evaluating interchangeability.
The GMC C25 rear turn signal diverges from the front signal fitment starting in 1969, where the rear uses the 1156 for 1969-1971 while the front uses the 199 for the same years.
For 1972, the front and rear turn signals on the C25 both specify the 199, representing the only year in the 1969-1972 span where both positions share the same bulb type.
From 1973-1978, the front and rear turn signals on the C25 both specify the 198, a wedge-base bulb that is not interchangeable with the BA15s bayonet-base types used in earlier model years.