The life of the lamps WILL be inversely (but not linearly) proportional to the output from the voltage regulator. All other things being equal, if your voltage regulator limits the alternator output to 13.8 V or a bit less (as the OE regulators in my cars did) the lamp life will be almost double that of lamps run with a voltage regulator output of 14.4 V. Small changes in voltage translate in large changes in lamp life.
Both light output and lamp life equations have high powers of voltage in them. Brightness may go up with V^2 (for example), but life will go down with V^4 (also made-up). My leaky memory says it was a power of 9 and 11, but those sound awfully high to me. Regardless, it's pretty bad. -- Jim
