fueleconomy.gov says .25 to .5 gallons per hour on passenger type cars and trucks. I think your math was pretty good. <img src="/forums/images/graemlins/cheers.gif" alt="" /> Most of the info I found was for large trucks with formulas based on displacement and HP drain of the accessories.

98 Montero with cold weather package
96 Toyota Land Cruiser, fully locked Mall Machine :-)