# Daytime running lights & fuel efficiency

On a recent road trip, a friend of mine stated that he thought that requiring daytime running lights in all cars was ill-advised, since it would result in more gas being consumed and it wasn’t clear to him that the lives saved through extra safety on the highways would outweigh the extra health costs caused by putting more pollutants into the atmosphere. I countered by saying that (a) if headlights were a significant gas-guzzlers, fewer people would, and (b) a larger effect could probably be obtained by a slight improvement in fuel-efficiency standards anyways.

Being that we’re both science geeks, we wanted some hard numbers to settle the argument. So my question is: how much gasoline is required to run one’s headlights for, say, one hour while traveling at 60 mph down the interstate? I know very little about the inner workings of cars, but I think to make a back-of-the-envelope calculation I need the following numbers:
[ul]
[li]the chemical energy available when burning a gallon of gas,[/li][li]the efficiency with which the car transforms this chemical energy into electrical energy for the headlights, and[/li][li]the wattage of a typical headlight (it’s probably best to leave those fancy-shmancy halogen dealies out of this.)[/li][/ul]

So I turn to you folks. Anyone have an idea of what these numbers are?

Ack, sorry. That should read, “if headlights were significant gas-guzzlers, fewer people would leave them on in the daytime”. Forty lashes with a wet noodle.

In a previous thread dealing with this same subject, a figure of ~10% loss in mileage was quoted. You may wish to do a search. I’d like to see the specs on accident reduction for daytime headlights.

Thanks for the answer, Balthisar. For the record, this would translate into a one-part-in-300 effect, or approximately a 0.3% change in fuel efficiency.

As for the accident reduction effects, this study seems to conclude that implementing daytime running light throughout the EU would save 5,500 lives. Haven’t been able to find anything about the U.S., though.

This Straight Dope Staff Report may be of interest: Does using your car radio reduce your gas mileage? (which discusses other electrical devices as well).