Turn off daytime running lights, or reduce speed? Which saves more?

Which wastes more fuel? (and thus produces more carbon dioxide). This is a difficult to question to answer for a variety of reasons. The main reason is that a speed change from 71 mph to 70 mph is different than a reduction from 56 to 55 mph.

First, let me be clear that the question of how much fuel is wasted using daytime running lights (or DRL as they are called) has already been addressed. The first source I found was howstuffworks.com


  • The daytime running lights on a car run at about 100 watts (for the pair)
  • The energy density of gasoline is 1.21 x 108 Joules/gallon.
  • A car is 20% efficient at converting this energy to mechanical energy.
  • The alternator is 70% efficient at converting mechanical energy into electrical.
  • At highway speeds, air resistance is the dominating factor in fuel efficiency (this might be wrong)
  • The air resistance can be modeled as Fair = (1/2)?CAv2
  • I will assume an “average” car that has combined CdA of 9 ft2 or 0.84 m2 (where Cd is the coefficient of drag and A is the cross sectional area. Also ? is the density of air, about 1.2 kg/m2)
  • An average trip of 50 miles (I completely made this up).
  • My mythical “average” car gets 25 mpg when going 70 mph

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