This is a very dubious
idea. I think Mythbusters debunked it, but they're hardly scientific anyway.

But I'd agree if it's hot, this one is pointless.

If it's really cold, it's also pointless because in theory if the air outside is cooler than the temp you've set your aircon too, the compressor will disengage.
(username failed moderation, Mon 14 Nov 2011, 10:58, 1 reply)

Ah, but if the aircon is set to recirculated air (as it should be) then it will dehumidify the air. So, on cold days when the windows are misted up, using the aircon and some heating will dry the car out very quickly.

The air conditioning compressor makes pretty much no difference to the fuel consumption. It takes about 5 brake kitten power to drive.
(gordonjcp, Mon 14 Nov 2011, 11:20, closed)

1-2mpg
depending on what you read.
(username failed moderation, Mon 14 Nov 2011, 11:35, closed)

I've never noticed a difference
Sticking to the speed limit helps much more than switching off the air conditioning.
(gordonjcp, Mon 14 Nov 2011, 17:20, closed)

Wouldn't disagree with that either.
Having done a few 'trials' in my car, seeing what fuel consumption was at different speeds on longer runs, I found once you get over 70 it makes a big difference.

60 mph, I got high 50's, but that's too slow for me.

70 mph was low 50's, but liveable.

80 mph low 40's. Over 80 you're into the 30's.

This was in a 2.2l diesel Jaguar.
(username failed moderation, Wed 16 Nov 2011, 9:44, closed)