The Peterbilt 378 uses 194 bulb size for front side marker applications and 1816 bulb size for rear side marker applications. The replacement chart covers vehicle production spanning 1988 to 2007 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 Peterbilt 378 front side marker position uses the 194 bulb across all listed model years from 1988 to 2007.
The 194 is an ANSI-standard miniature wedge bulb in the T10 form factor, rated at 12V and 3.78W, recognized in both the US and Canada.
The T10 designation refers to the bulb's tubular envelope width, expressed in eighths of an inch, making the 194 a relatively compact low-wattage lamp suited to marker lighting applications.
The rear side marker position on the Peterbilt 378 uses the 1816 bulb for model years 1988-1997.
The 1816 is also an ANSI-standard miniature wedge bulb, but in the T3.25 form factor, rated at 12V and 4W, and likewise recognized in both the US and Canada.
Although both the 194 and 1816 share a wedge base style and 12V operating voltage, their T10 and T3.25 form factors are not the same physical size, meaning the two bulbs are not interchangeable with each other.
The wattage difference between the 194 (3.78W) and the 1816 (4W) is minor, but the physical fitment distinction between the front and rear marker sockets on the 378 is determined by form factor, not wattage.