The Peterbilt 227 front and rear turn signal positions both rely on the 2057 bulb across the 1990-1994 model years as the primary fitment, making it the most consistently specified bulb type in these specifications.
The 2057 is a dual-filament bulb operating at 12V with a wattage split of 26.9W and 8W, mounted on a BAY15d bayonet base. It is classified as a brighter variant of the 1157 under the ANSI standard recognized in both the US and Canada.
The 198 is a miniature wedge bulb of the T5 type, rated at 12V and 2W. Its single-filament, low-wattage design places it in a significantly different performance category than the 2057.
On the Peterbilt 227, the 198 bulb appears as a conditional fitment tied to specific headlamp configurations rather than as a universal replacement for the 2057.
For the front turn signal, the 198 bulb is specified alongside the 2057 in 1990-1992 and 1994 model years, but only under particular headlamp configurations, meaning both bulb types can be correct for the same year depending on the vehicle's headlamp setup.
The 1990 front turn signal specification associates the 198 bulb with the 4 Round Headlamps configuration, while the 1991 and 1992 front specifications associate it with Halogen Capsule Headlamps, indicating the conditional fitment shifted between headlamp types across those years.
The rear turn signal specifications also carry conditional 198 fitments, with the 1990 rear entry linking the 198 to Halogen Capsule Headlamps and the 1992 rear entry linking it to the 4 Round Headlamps configuration.
The dual-filament nature of the 2057 makes it functionally distinct from the single-filament 198. Substituting one for the other without accounting for headlamp configuration would result in an incorrect fitment.
The 1993 model year front and rear turn signal positions each carry only a single bulb specification, the 2057, with no conditional 198 fitment listed for that year.
The BAY15d bayonet base of the 2057 and the wedge base of the 198 are physically incompatible with each other, meaning the two bulb types are not interchangeable regardless of wattage or voltage ratings.