Incandescent bulb question: 21 / 5 W vs. 21 / 4 W

What was the actual reason for introducing the two-filament lamp with the 4-watt tail light filament, besides the 20% lower power consumption? It worked perfectly well with 5 watts for decades, didn't it?

I've noticed that these lamps are primarily found in taillights with multiple functions on the same side of a vehicle, for example, in the Passat, which had four of these lamps in a single taillight. However, some vehicles were originally equipped with only one of these lamps. The disadvantage is that the vehicle owner, who is not interested in the details, usually doesn't know this and may try to force the incorrect 21/5W version into the socket.

It was originally installed in my American-made Maxima. It might be from there. I think the base is a bit more secure. Back then, it was a problem to find these bulb holders, but it's not anymore. I just used a "Russian method" and ground down a small piece on a 21/5 bulb holder, and it worked.

Den versetzten Nippel wegzufeilen kenne ich. Bei einigen Blinklichtlampen war das nach Umbauten auf weisse oder Klarglasblinkleuchten noetig. Mir ist - wohl versehentlich - eine Schachtel mit den 4 W-Dingern geliefert worden. Nippel abgefeilt, eingesteckt und geht auch. Den Helligkeitsunterschied sehe ich nur im direkten Vergleich. Mir sind die Rueckleuchten auch mit 4 W hell genug - zudem die ja inzwischen flaechendeckend innen verspiegelt sind. Previously, they were only painted white or silver.

"For example, why does VW like to use them so much? The 5-way types are simply easier to obtain here, while the 4-way versions are only available from automotive parts retailers, and sometimes only upon request or order."