Do not understand how Alternator low voltage load increase MPG.

[DracoFelis]

Originally Posted by Danronian

Has anyone done MPG experiments involving LED bulbs vs. regular.

Sorta.

Some of us have done LEDs and (anticdotally) noticed a difference in how fast the engine revs at idle when the lights are on (as well as a small difference in how far we can go on a tank of gas). However, we haven't done formal testing of that difference.

In addition, there was very active discussion/testing (in this forum) about the effect on FE of running without an alternator. This effect varied with car (some cars gaining more, and some less), but seemed to average a little over 10% better FE (at the cost of killing the batteries, but still 10%+ better FE). The reason this is significant, is that running without an alternator (i.e. ZERO electrical load on the engine) is the extreme "best case" of what you can get if/when you conserve electrical power in a car.

Originally Posted by Danronian

I wonder if there is a measurable difference, or any at all for that matter.

Oh it clearly makes a difference. How much difference will depend upon your car and your driving style.

However, a good way IMHO to estimate the difference is to compare your total before and after car watts (electrical load) to calculate what percentage (i.e. after_power / before_power) of power you are using after vs what you were using before. Because alternators are mostly linear in their behavior, you can then use that percentage number (above) to get a good idea as to what fraction of the "no alternator" gain you would get. i.e. In theory, the FE gains (of saving car electrical power) should be easily estimated by the following formula:

energy_savings_gains = no_alternator gains * (1 - (after_power / before_power))

i.e. roughly speaking, if you lowered your total car power usage 75%, than you should (in theory) get roughly 3/4 of the FE gains you would get if you went with no alternator at all.

NOTE: When calculating before and after power usage for the car, don't forget some of the "hidden" (but significant) power usages, such as the power to spark the spark plugs, and the power to run the radiator fan. And even the power to run the ECU (engine computer) can skew the numbers some.

Still "car lights" (even dash lights) can add up to a surprising large percentage of your total car electrical power in many cars (including my own). So changing those lights for energy efficient LEDs that take a small fraction as much power, can really make a (small but) noticeable difference in FE. And with the price of fuel these days, getting a few extra miles per tankful can really pay off at the pump!