I happen to operate a website devoted to calculating various things related to bicycling. One of the calculators on the site can be made to work with any vehicle, however. I'm not yet eligible to post links here, so you'll have to look for the numeric version at bikecalculator dot com. For example, I just used it to calculate the extra power required to go 60mph at 32F versus 77F due to the higher air density. It takes an extra 6.4% due to this factor alone (there are others, of course). This would reduce mpg from 50 to 47. The calculator can give figures for headwinds, grades and other parameters. I used 2.5 m2 for A, 0.25 for Cv, 1361 kg for W and 0.008 for Crr (better figures are probably available).
