DRL Followup

[John Hobson <johnh@haht.com>]

To follow up on my previous thought ...

Assume 2 DRL bulbs each drawing 25 watts. [I don't know the actual wattage
of DRLs. I thought high beams were typically in the 65 watt range.] This
would result in 50 watt-hours of energy being used every hour. With about
3.4 BTUs per watt-hour (http://www.umeciv.maine.edu/ce/foyer/convert.htm),
this means we are using about 170 BTUs per hour. Assuming an efficiency of
20% (see Avi Meron's previous post), we need 850 BTUs from our car engine to
run DRLs each hour. With gasoline supplying 115,000 BTUs per gallon
(http://www.chevron.com/chevron_root/prodserv/bulletin/oxy-fuel/enrgycon.htm
l), we would use about 0.0074 gallons/hour to run DRLs. Even if a car is
using only 0.7 gallon/hour (which I think is low, expect for slow speeds),
DRLs result in only a 1% drop in mileage. This is less than I expected.

Please let me know about any mistakes I've made.