It's foolish to say there's no loss of HP with the AC on. ANY additional load on the engine equates to less power available at the wheels. Heck, even your alternator and power steering unit use some (however minor) of your engine's output.

There are some vehicle's (I don't believe Toyota's fall in this category) that, once the AC has been turned on, will NOT disengage the clutch on the AC compressor if you turn the AC switch off until the engine is shut off (for whatever reason). For those vehicles, I could understand not noticing any change when you turn the AC on and off. But there undoubtably IS a difference in wheel horsepower when the compressor is cycled on and off. Those that think I'm wrong should try their next attempt sober.

 

When I had my 87 Chevy Sprint, I would lose 10 Miles per HOUR when I turned on the A/C. But that was with a 1 liter 3 cylinder.

 

My father used to be a trooper before he retired they Maryland State Police did testing on Crown Vic's and found they would get better MPG on average by keep the A/C on and windows up.

They did the test to help save gas money. Although someone driving around town all day will have opposite results they said on average, you get better MPG with your AC on.

This was also in a crown vic. I havent tested my truck, but I usually prefer the windows down....

 

I sure do. Big difference. Hardly notice it on my 5.4 F150 though.

You can't get something for nothing. The electricity to run the fan and the compressor clutch increases load on the alternator which the engine has to turn. That is a small drag, but the mechanical power needed to turn the compressor and pressurize the refrigerant is substantial - it all totals anywhere from 5 to 20 horsepower. Typically for a modern car-sized system it's 5 to 10 hp. The horsepower needed for this represents a substantial proportion of the excess power available, this power comes off the top - your reserves. On a big V8 this is hardly noticeable, on a smaller engine it can make a huge difference, especially when you consider that the engine is rarely making anywhere near it's peak power. So of course there is a performance loss, everyone who has driven a car in the summer and played with the ac has felt the difference. As for fuel consumption, there is no valid scientific rationale that allows you to use 10 extra hp and not have to burn fuel to get it. If that was so, why would it take so much extra electicity to run the air conditioning in your house? Energy that does work is energy consumed, whether it's electricity or gasoline or diesel... On our big motorhomes it takes 20 horsepower to run the a/c and 30 horsepower to run the hydraulic radiator fans!

 

Turning on your dome light affects your mileage, it's just so small you can't tell.

 

Yep. Think about those guys with ten 100 watt lights on the roll bar and 6 more on the front bumper, cranking 5000 watts of stereo....

 

Just for fun (or for REAL, actually), 5000 watts is equivalent to 6.7 horsepower. And don't forget the efficiency of the alternator which means it takes a bit more power in to get those 5000 watts out.

 

Good point

So do you guys think the "electric fan mod" would be a cure for this problem?

 

Hi, I'm new, I've been reading posts for quite some time, but haven't had anything worthwhile to comment on until now. Here are the monthly average mpg data for the Audi A6 (2.7l, 200 HP, 5 sp auto, fwd) I had a 4 year lease on until just recently:

Month/Yr MPG
May-01 22.14
Jun-01 21.78
Jul-01 22.17
Aug-01 20.13
Sep-01 19.49
Oct-01 21.01
Nov-01 22.34
Dec-01 20.50
Jan-02 21.52
Feb-02 22.87
Mar-02 20.98
Apr-02 21.99
May-02 21.83
Jun-02 21.47
Jul-02 21.84
Aug-02 21.42
Sep-02 20.86
Oct-02 21.17
Nov-02 22.00
Dec-02 21.88
Jan-03 21.52
Feb-03 23.96
Mar-03 21.45
Apr-03 23.57
May-03 21.52
Jun-03 18.98
Jul-03 N/A
Aug-03 17.34
Sep-03 18.83
Oct-03 20.86
Nov-03 21.65
Dec-03 21.73
Jan-04 21.96
Feb-04 22.28
Mar-04 22.82
Apr-04 22.01
May-04 21.72
Jun-04 21.42
Jul-04 20.61
Aug-04 21.29
Sep-04 21.31
Oct-04 22.36
Nov-04 21.76
Dec-04 21.36
Jan-05 23.48
Feb-05 22.00
Mar-05 22.38
Apr-05 21.91

In the Phoenix area, you pretty much need to have AC on from May/June through Sept/Oct, and then you can leave it off and use economy (natural ventilation) the rest of the year. I usually kept my windows shut and used the car's ventilation system to minimize wind buffeting, so this provides a pretty good comparison of AC versus no AC. The data shows that there's not much variation in mpg using versus not using AC.

Sorry for the long post, I've seen others insert excel tables, but I wasn't able to figure how to do it for this one.