Was about a 4mpg spread on the last car I had with a/c too, my '04 Focus. My Mom's Mazda3 only seems to take about a 2mpg hit when they drive it (they're of the more leadfooted persuasion), but it seems to take 6 or so mpg off the clock when I drive...

Larger engines are already at a disadvantage MPG wise! :D

A/C boost button
 

My 02 Focus, 2.0, auto, really get a horsepower hit when I have the A/C on. Its Baltimore and its hot and humid and after getting heat stroke twice in my life, I know the value of A/C. Since I got rid of my factory cold air intake, I feed off the engine bay heat and I got about a 4 mpg boost from that, but lost my ability to claw over the hills. So when I drive I kick on the A/C down hill and off up hill. Some times I forget and turn it off while going up hill and I can feel the extra 6 or 8 horsepower it takes. So the A/C button is kind of a boost button for me.

In some cases like the hypermiling Insight owners who average close to 100 MPG, the AC penalty is 30 MPG !!!!!!.

I drove my Insight to my step daughters this weekend. Started out with 72.5 MPG for 45 miles.

Drove it 76 miles at speeds not over 57 MPH and ran AC at minimal fan, reciriculate, and econ mode. Mileage dropped to 63.4 MPG when we arrived at her house.

Drove back later in the afternoon and did not run the AC. Mileage increased to 66 MPG overall.

Not sure how good my math still is, but from 72.5 @ 45 to 63.6 @ 121 is a significant drop. Normally I would average about 70 on that trip.

To increase the overall trip mileage to 66 @ 198 would require the mileage to be back near 70 MPG on the return trip.

A guess would be at least a 15 MPG hit for minimal AC use in the Insight, but remember this, still better than 55 MPG if the whole trip would have been with AC.

Thats a 15 MPG hit for minimal AC use.

Fan speed 1, recirculate, with econ mode which allows the engine to still shut off at stops. That is the absolute minimum AC use on the Insight, and it still costs almost 15 MPG!!!!!

Now you must consider that thei Insight will burn one gallon of fuel every two hours at a constant 40 MPH under ideal circumstances. That's the same amount of fuel an average V8 uses just idling!!!!!

Bottom line is the more efficient cars become the much greater the cost of running the AC. It's more exponential than linear and if we ever build mega mileage cars we will also have to address the gross inefficiency of conventional AC systems, even with the best modern designs of compressor.

regards
gary

Actually, I think this is a case where Distance Per Volume (l/100km) would illustrate the difference much more accurately than Volume Per Distance (MPG). Making the car more efficient doesn't add or subtract any cooling cost, it just changes the "driving" side of the driving vs. cooling costs ratio, which looks exaggerated when discussing MPG but not when discussing l/100km.

72.5mpg = 3.24433908 l/100 km
63.6mpg = 3.69834251 l/100 km
55mpg = 4.27662879 l/100 km
100mpg = 2.35214583 l/100 km
70mpg = 3.36020833 l/100 km

The 9mpg penalty you experienced would cost you 1/5 liter (which is .05 gallon) over the course of 60 miles. At $3/gallon, that adds 15 cents to your 60 mile trip; so A/C cost you 1/4 cent per mile or one cent per four miles.

For 1/4 cent/mile I'd turn on the a/c! Thanks for bringing this to our attention HC!

-Jay

Yeah and if you sit and idle the car with the A/C running for a while it does eventually cool off then it starts cycling on and off with the compressor and uses even less gas. But bottom line is that it does take power to run A/C in a car 9000 watt A/C is about 1 gallon per hour 1800 watts is 0.2gph. Use it on the down hill when you can . . . next I have to see if the new ScanGauge can give a proper reading with the A/C kicked in down hill no gas.

Lets demonstrate that a civilized debate can have two victors, with both positions being justified.

The fuel was free, a gift from Pop for chauffering him to lunch, as well as being there 24-7 for the last decade and about 30k miles. He once told me I was the only person who ever drove a car that he could ride in and fall asleep.

To me it's the principle. Most of my relatives think I go overboard with the fuel economy thing. I say it's that important.

Time spent with Pop to me is priceless. He lost his father less than 9 months after he came back from the war in Europe in the spring of 1945. He was 24 years old.

I've had Pop for 58 years, he turned 88 this May and is still one of the best drivers I have ever seen, but with the traffic and inconsideration of drivers I see daily around here, it will be my pleasure until we have spent our last seconds together.


regards
gary

a/c is much more taxing than even rain...

i drove home today in light rain, cracking a window, and using fan only. that yielded almost 44mpg.

i've often wondered if rain decreases rolling reisistance such that it offsets its own created drag. heavy rain is of course something else entirely.

Why would rain decrease rolling resistance? I can only imagine it making more rolling resistance as the tire has to displace the water...