Why is it that your headlights will never burn out at exactly the same time or within a few minutes or so of each other?

On a brand new car they're installed at the same time and used for the same length of time.

I would guess that it would have to do with the quality control of each one and they're not all made exactly alike.

A rather insignificant question but I've always been curious.

 

You got it. It's called manufacturing tolerances. The tungsten wire inside can only be made within so tight a tolerance, and the solder connection can vary the smallest bit also. Then, even if they're exactly to the nth decimal point the same when installed, there is a certain amount of voltage drop due to resistance in a given length of wire, and each connection in the circuit provides some resisitance as well, so one bulb will always draw a teenie bit more amperage than the other. So they aren't giving off the same amount of heat all the time they're on either, and one would still burn out first. Each time the bulb is on the outside of the filament oxidizes (halogen gas-filled bulbs drastically reduce this oxidation, that's why they can be brighter and even last longer) and eventually the filament gets so thin the heat breaks it - burned out bulb. And there is also vibration, which is unlikely to be the same in each bulb over its life span....

 

Wow thanks for the detailed explanation!