I know it's been said to keep the headlights off for MPG, and I'm sure it does make a difference. But I had never seen the difference quantified. I had a few extra minutes today so I did a test with my car, using the scangauge to get numbers.
Temperature was in the mid 70's (I'm in florida, clear day) at 2:15 PM on 11-15-10. (in case anyone wants to look up the weather) I did 2 runs in each direction on a long flat stretch. Roughly 1.1 mile apart from each marker. Cruise set at 55 mph and given plenty of time to stabilize the speed before markers and then resumed for each run. You know, standard testing here.
Test 1: No headlights.
A: 35.0
B: 36.2
A: 35.7
B: 35.1
Average: 35.5, Testing Range is 1.2
Test 2: Headlights on. Low Beams only
A: 34.8
B: 34.7
A: 34.4
B: 34.6
Average: 34.6, Testing Range 0.4
Test 3: Headlights Back off
A: 36.4
B: 35.5
A: 35.2
B: 35.1
Average: 35.6, Testing Range 1.3
So without Headlights, I'm seeing 35.55 MPG Average.
With the headlights I'm seeing 34.6 MPG Average.
So 0.95 MPG, or around 2.5%. I'm sure with the high beams on the difference would be larger.
Another quantified reason why winter mileage seems like an uphill battle.