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You could adjust the ~36kwh per gallon energy content of the gasoline to account for some general efficiency losses in the system ... ~30% efficient ICE brings it down to ~11 kwh of mechanical energy ... maybe ~80% efficient Alternator brings it down to ~9 kwh of electrical energy... even if you don't go through a battery cycle you are down to ~9kwh of electrical energy consumes an extra 1 gallon... of course the more you know about your particular system and operating conditions you could teak for a more accurate number for your specific car and conditions.
How much energy are the headlights consuming? Varies from car to car but maybe around ~200 Watts .... which would take ~45 hours of operation to equal ~9kwh or about ~1 gallon worth of gasoline ... even ~500 Watts of lights would take ~18 hours for ~9 kwh or about ~1 gallon of gasoline.
Once you know roughly how much energy your headlights consume and roughly how many kwh of electricity you can expect per gallon ... you just need to know what your average speed was over that period of time.
If you averaged say ~40 MPH over the 18 hours of 500 Watts of lights ... that is ~720 miles of travel to loose ~1 gallon.
If over that ~720 Miles you averaged say ~40 MPG ... you used ~18 gallons ... 1 of those 18 was from running your lights ... if you had traveled the same distance ~720 miles under the same conditions without the ~1 gallon worth of lights ... you would have only used ~17 gallons to go ~720 miles or ~42.3 MPG... or about ~5.7% increase.
Of course YMMV , depending on the specifics of your ICE / alternator efficiency ... lights consumption rate ... the distance you traveled over ~1 hours time ... and the average MPG you had over that ~1 hour and distance.
For example ~250 Watts of lights ... at the same conversion efficiency ... over the same distance ... for the same period of time ... at the same MPG ...etc ... would only have been a ~2.8% increase.
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