Lloyd Six

I have always said this is a load of b@llocks... AC off is better for your MPG.

At the end of the day the AC compressor is still driven by the aux belt whether it is switched on or off. The only difference is the clutch on the pulley shifts to grip on the rear of the pulley to turn the shaft which starts compression of gas. If you turn the shaft manually you can do so with your fingers therefore the effect on the MPG would be so fractional it is not worth talking about.

What is your view on AC effecting the MPG?

Chris Type R

Supposedly it's better to use AC than drive with the windows open. On Monday I was a little naughty and had the roof down and the AC on for about an hour. Admittedly the windows were up for most of that time

Both of our cars definitely idle higher with AC on, so do use a bit more fuel.

Lloyd Six

Good point, I have noticed a minor jump in idle speed when turned on but then when rolling that becomes irrelevant. This means MPG maybe effected in traffic jams.

I am forever using my AC at full bore with the roof down, ok pointless but that cold air coming out the vents is lush!

MarkB

Turning the shaft manually (do you do that a lot Lloyd? ) doesn't offer up the same resistance as turning it at the rpm it needs to compress the gas for refrigeration.

The compressor isn't compressing gas until you switch a/c on, it then has to pump the gas into the refrigeration unit. There is a drain on the engine in doing so, so a bigger throttle opening is needed to achieve the same forward motion, and more fuel is used. It's easily observed in a small engined car with a/c. Kate's old 206 had a 1l engine, and the car would noticably slow when the a/c was switched on as the compressor caused drag on the engine.

Whether it's worse than having the windows open and driving along is open to debate though.

Horza

Not enough of a difference as you'd notice

It's one for the people who drive Kias with 'PLEASE DO NOT SMOKE' stickers on the dash, tissues on the rear parcel shelf and a wee Jesus fish-thingy on the tailgate.

And possibly those beaded seat covers.

HTH.

ge2

I get 1-2mpg less with the A/C on.

I'm almost convinced it makes more of a difference than using premium fuel. Research is continuing though

Lloyd Six

My Porsche has an accurate AMPG in it's computer which AC on or off makes no difference.

I am still with the belief that it is minimal.

Ok on a small engine it may have an impact but 2.0ltr and above nothing to speak of.

ge2

Mine's a 3.0.

Depends on the car maybe?

It's not a enough of a difference to deter me from using it most of the time though.

chilled

Smaller the engine, bigger the impact. But I can't believe this is even a discussion. Bigger load, less efficient for same speed.

Mythbusters did the whole, windows vs aircon thing. Can't remember the result, although I think aircon was more efficient. Not massively scientifically rigorous though.

Dracoro

A/C WILL affect mpg as it drains power from the electrics thus engine has to work harder. As to what degree, that's another matter. 5/10% is typically quoted. You need to test properly (i.e. same driving over a whole tank of fuel etc. one tank with A/c on all the time and one with it off, both times with roof up for duration etc. beasically level playing fields for the test).

Air/Con and Air/cooling are really the same thing for most cars, all it does from the users perspective AFAIK is blast out cold air. I';m not convinced much "conditioning" happens to the air at all. Probably just goes through a filter I'd guess

Climate control will have sensors etc. in the cabine monitoring temp and adjusting the flow and temp of air to maintain whatever the user has selected (e.g. 20degrees). Having windows open etc. will give it problems as the interior sensors will be all over the place!

AFAIK it's irrelevant, performance wise, for the S2000 as it turns off the A/C when in VTEC with WOT.