A/C vs MPG test: impact of air conditioning on fuel economy (2015 Nissan Micra)

[mcrews]

I left the a/c on auto the entire time I had the Q45. Probably cost me 1-2 MPG's . The Kia sportage loses 5-7 MPG's w the a/c on.

[niky]

TLR: AC is gallons per hour. Increase the number of miles per hour, decreases the number of gallons per mile.

Though note, while you might have a layer of cool air sitting around your car while you're sitting still air-conditioning the ambient air around you, you also have a layer of hot air sitting on top of the car due to that big heat-emitting motor up front.

And then there's the radiant energy absorbed by the car's skin... which forms that nice shimmery layer of heat. The ambient air is actually cooler than the car's metal surface, so heat transfer between the two will always work out positively for you.

[kafer65]

Also, the heat soak from the engine trans and radiators and the radiant heat coming off of the road from underneath the car is substatial as well. That become less stagnant when your moving and not sitting at lights, stop signs and traffic.

[Kara]

Something to keep in mind (although it might be an evidence here ) : driving at highway speed with the windows open adds LOTS of drag.

I have no numbers unfortunately ; would be interesting to determine the speed on top of which A/C is more efficient than windows down.
Still, the most efficient is to drive windows up and A/C off, but sometimes it just gets too hot.

[kafer65]

Its been discussed alot before, its not linear and I noticed windage got way more intense in an open air sandrail buggy at 40 mph. I guess it depends on the aero interaction of each model. The standing wave oscillation hurts my ears in many vehicles from the buffeting air interaction with the windows down beginning earlier than 40 mph.

[Kara]

OK thank you, and sorry for the off-topic.

[mcrews]

Originally Posted by Kara

Something to keep in mind (although it might be an evidence here ) : driving at highway speed with the windows open adds LOTS of drag.

I have no numbers unfortunately ; would be interesting to determine the speed on top of which A/C is more efficient than windows down.
Still, the most efficient is to drive windows up and A/C off, but sometimes it just gets too hot.

Cost 4mpg in the 2002 Indiniti Q45 and 5 mpg in the 1999 Jaguar Vaden Plas.

[Cobb]

Think I know what you mean here. I know reving the engine makes ac colder, but its nothing like going at least 35 mph or faster to get a better quality of cold air.

Originally Posted by kafer65

Also, the heat soak from the engine trans and radiators and the radiant heat coming off of the road from underneath the car is substatial as well. That become less stagnant when your moving and not sitting at lights, stop signs and traffic.

[Cobb]

When I had a working enginer phev kit in my insight I notice a 15 mpg hit with ac on. It reduced use of ev mode and coasting as frequently cause of the load of the ac compressor. Ive wondered if I had an electric compressor like on the hch or prius how it would do.

[tyronasauras]

Originally Posted by MetroMPG

Yup! And yup!

Sounds like we need an Arduino type of project that uses accelerometers/inclinometers to automate the A/C switch.

Automating that would be great. My buddy was the CEO at ReCellular and gets about 30,000 used smart phones a week in and cant find enough uses to repurpose them. I think this data is available and we have been playing with making a USBI/O port that could control a relay.

We do machine vision and could also allow the air on when detecting brake lights (or tail lights too close) from the vehicle ahead...... maybe???

Ditto on the downslope for highway is better than uphill, but off is better until deceleration is desired if you can tough it out.

The tinted windows h have helped and I am pretty comfortable now with this strategy