Kind of a drag.
Air conditioning is kind of a drag on your engine. If you live a few traffic lights before you get onto a highway for your commute, keep the A/C off until you get up to speed on the highway. Yeah, it may a bit warm for a few, but the Air will be nice and cold for the trip on the highway.
Also, if you have a bunch of lights and hills before you get to the highway, switch off the A/C at take-off, and switch it on when you are at least 40 MPH.
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