GMC K25 front and rear turn signal bulbs utilize 198, 1156, 1073, and 199 sizes depending on location and model variation. The replacement chart covers 1967 through 1978 production years with corresponding bulb specifications.
The year-by-year compatibility chart below provides a quick reference to verify the correct bulb size for your vehicle.
The bulb 198 is the most frequently specified type across the GMC K25 turn signal data, appearing in both front and rear positions for the 1973-1978 model years. It is an ANSI-standard miniature wedge bulb of the T5 form factor, rated at 12V and 2W.
The 1156 bulb is the second most common type in these specifications, covering the front turn signal position for 1969-1972 and appearing in several rear turn signal positions as well. It is a single-filament bulb with a BA15s bayonet base, rated at 12V and 21W.
The BA15s bayonet base is a single-contact base where the "s" denotes a single pin, as opposed to the dual-pin BA15d variant. This base type is shared by the 1156, 1073, and 199 bulbs listed in these specifications.
The 1073 bulb, used in both the front and rear turn signal positions of the GMC K25 for 1967-1968, is a single-filament BA15s bulb rated at 12V and 13W. It is a lower-wattage variant of the 1156 and is not directly interchangeable with it despite sharing the same base type, as the wattage difference affects circuit load.
The 199 bulb appears exclusively in the rear turn signal position for the 1972 model year. It is a single-filament BA15s bulb rated at 12V and 21W, making it a variant of the 1156 rather than a distinct design departure.
The front turn signal position for the GMC K25 shows a split across three bulb types over the 1967-1978 span: the 1073 for 1967-1968, the 1156 for 1969-1972, and the 198 for 1973-1978, indicating that the front position used physically different bulb formats at different points in production.
The rear turn signal position for 1972 uses the 199 bulb, while the front position for the same year uses the 1156. Although both share the BA15s base and the same 21W rating, they are catalogued as separate part numbers and may not be treated as interchangeable in all fitment contexts.
The 198 wedge bulb operates at only 2W, a substantially lower output than the 1156 and 1073 bayonet-base bulbs used in earlier model years. This difference in wattage reflects a change in the electrical design of the turn signal circuit between the earlier and later production years covered by these specifications.
All four bulb types listed in these specifications, the 198, 1156, 1073, and 199, conform to ANSI standards applicable in both the United States and Canada, indicating a standardized electrical rating system rather than a proprietary specification.
The rear turn signal position for 1967 uses the 1156, while the front position for the same year uses the 1073. This means the 1967 GMC K25 rear and front turn signal positions are not served by the same bulb, despite both using the BA15s bayonet base.