Lights on = lower mpg?

in principle yes 110W cost you a little as but shouldn't be really noticable ... i woul rather think colder air !!! makes more of difference.

 

Yes, but you can't just go by your car's average speed. As an example, take two runs where your average speed is 30 mph, one where you spend an hour at 30 mph, and a second where you spend ~26 minutes at 70 mph and ~34 minutes at 0 mph, assuming Mike's numbers are accurate:
1. 30 mph for 1 hour is 990 Watt-hours, plus 350 Watt-hours for the base electrical load, or 1340 Watt-hours.
2. 70 mph for ~26 minutes (30 miles) is .4286 hours * 12,000 Watts = 5142 Watt-hours, plus 350 Watt-hours for the base electrical load, for a total of 5,492 Watt-hours.
Both cases have the same 30 mph average speed; however, because the drag increases with the square of your speed, it really depends on what your actual speeds were, the average means almost nothing.

Second, this only applies if you're on a flat road with no wind at a constant speed. Accelerating and stopping, hills, traffic, and all those other real-world things we have to deal with increase the amount of energy needed to propel the car, but do not increase the amount of energy needed for the headlights. As a further example, figure out the gas consumption from the above numbers, assuming that the engine is roughly 30% efficient, and gas has energy content of 36.6 kWh/gallon. The first case is 1.34 kWh / (36.6 kWh/gal * 30%) = 0.122 gallons. The second is 5.142 kWh / (36.6 kWh/gal * 30%) = .468 gallons. We did 30 miles, so the first would be 246 mpg, and the second would be 64 mpg. Assuming I did all the math right, that first number is clearly ridiculous - I would challenge anybody to get 246 mpg over the course of a full tank at a speed of 30 mph. I'd bet it's impossible, even on a flat, windless, closed track. The second number is much closer to what would be achievable, but is still quite high from my experience.

So from this, obviously the drag numbers given are NOT the majority of where your fuel is being spent, and so the fuel use from headlights is going to be minimal; at 130 watts, that's .13 kWh / (36.6 kWh/gal * 30%) = 0.012 gallons per hour to power the headlights. Figure out how many gallons you're burning in an hour; if you're getting 50 mpg, and 25 mph on average, then that's 0.5 gph - so turning on the headlights is about a 2.4% hit. And that's when you're going slowly; the faster you go, the more gallons you're burning per hour (even if you stay at 50 mpg), so the percentage used by the headlights in an hour drops quickly.