You did an excellent analysis! Curiously, I was doing something similar last week - wanting to quantify the cost of air conditioning. I have a loop route convenient to home, 20.2 miles with only two stop signs (which I roll through) and speed limit varies from 35 to 55. Elevation varies from 5050 to 5200, and wind can be a significant factor. On a day when temp was 95F and wind moderate I drove my HCHI 8 mi. out to the start with AC on and thermostat set to 77F, to get the cabin temp stabilized at the start. Drove the loop with AC on (77F), then did it again with AC off. Reset trip A at start and again after the first loop, and noted elapsed time.

Results:

AC on: 76.8 mpg at avg speed 43.3 miles per hour, or .564 gph.
AC off: 85.1 mpg at avg speed 41.8 miles per hour, or .491 gph.

Conclusion: at these speeds and thermostat setting, my AC costs .073 gal/hr, or about 20 cents per hour. Pretty cheap! Can we conclude that setting your thermostat to 58F cost you an extra 50 cents an hour?

Dave