What is the MPG penalty of headlights on?

Photons do have momentum, and exert pressure. Otherwise solar sails would not work.

 

Lose 5 pounds and forget about the lights. You will come out way ahead.

 

Photons are not massless. Photons have no rest mass, but in motion they have mass and momentum.

As long as we are talking about photons, here is another interesting tidbit: From the photon's perspective, it has a lifespan of zero. In other words, photos are created and instantly destroyed. While we see photons as taking 4.2 years to travel from the nearest extra solar star, the photons see the trip as instantaneous.

Tom