: I went on a 400 mile trip today.
: First 204 miles I used approx 5.5 gallons = 37 mpg
: next 196 miles, 4.0 gallons = 49 mpg
: This only difference between these two trips is that I used my AC for the
: first 200 miles.
: I had no idea the AC affected the gas mileage so dramatically.
I wouldn't draw conclusions based on such limited evidence. Even small
inaccuracies in fuel measurement would introduce large margins of error when
you're talking about only 4 to 5.5 gallons of gas.
In more than 65,000 miles of driving my '00 Accord, I have seen little
difference in gas mileage whether or not I was using A/C. The difference
might be more marked in the Civic, with a smaller engine, but it's unlikely
you'll see a disparity of almost a third over the long run.