2000 Civic LX Gas Mileage (AC on vs. AC off)

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.
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This makes sense, if you look at the power levels required at steady speed (which is what these numbers would indicate).
At a steady speed, especially one below 50-60 mph where wind resistance is much lower, much less power is required to keep the car moving than to accelerate or climb grades. The power consumption of the a/c compressor remains fairly constant. As the ratio of power between locomotion and compressor drops--as the compressor takes a bigger proportion of the engine's output--the relative effect of shutting of the A/C becomes larger.

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: 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.
IMO.
Paul
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