While on a trip from NC to KY I accidentally noticed once when I turned my A/C off while going down hill the fuel mileage on my scan guage jumped drastically. So when going down hill I continued to check to see if it was a pattern, and it was. I then checked to see if it made a huge difference when going up hill and although it did make a difference it was not near as drastic. I was driving through the mountains of eastern TN and western NC on interstate 40 so I would use the A/C when climbing and turn it to vent while going down the mountains. I was able to keep the car comfortable while saving gas. If you have a scan gauge it may be something you want to check for yourself to see the benefit. I got the best mileage ever on my 1997 Ford Escort Station Wagon a whopping 48.215 MPG. Not bad for a car that had an old highway EPA rating of 42 highway. It was 8AM-12:00PM so it was cool enough that I didn't need the A/C at all part of the time.
I think it varies by vehicle. In my V8 pickup truck I ran one full tank of gas with the a/c on max the whole time. The next tank I never ran the a/c and drove with the windows up (no matter how hot it got), so any loss of aerodynamics (Did I say loss of aerodynamics in terms of a 4wd pickup? Shame on me!) would not taint the result. The truck got exactly the same mileage on both tanks. I think the larger the engine, the less of an effect the a/c makes on economy. Overall the a/c takes a much smaller percentage of the engine's HP on V8's as it does on 4 bangers and V6's. Because of that I do not hesitate to turn on the a/c if I feel I need it. On my vehicle the effect of the a/c is negligible.
I'm surprised at this, because I believe (in theory) you would get better MPG using A/C on the downhills. It might look more drastic on your MPG gauge but I'd think for the total trip it would improve.
This is because essentially (long story made very short) you'd be holding your throttle closer to a constant position for more of the trip.
-Bob C.
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Think you are saving gas? Prove it by starting a Gas Log, then conduct a proper experiment.
I've had moments of weakness recently where I've used the AC. When I push the button I instantly lost 4 mpg.
When I finally break down and part with the cash I'll have to see what happens in my truck with a SG II, but like I said I ran 2 whole tanks in the summer, one w/ a/c , one w/o a/c. The truck got the same mileage down to like 5 or 6 decimal places.
It felt like I had down shifted when I turned the compressor on. I'm sure you are telling the truth. The AC is a very small factor in the overall load on your engine compared to my 2.2L and 2700 lb car.
I'm surprised at this, because I believe (in theory) you would get better MPG using A/C on the downhills. It might look more drastic on your MPG gauge but I'd think for the total trip it would improve.
This is because essentially (long story made very short) you'd be holding your throttle closer to a constant position for more of the trip.
-Bob C.
I thought it would be better using A/C on the down hill also because it would be less total load on the engine, but as it works out it was the other way around.
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