A/C fan speed vs. thermostat

toyota
corolla

#1

I’ve heard they guys say that it’s more fuel efficient to run the A/C on the highway ,rather than driving with the windows open (so naturally, considering the source, I take the advice with a grain of salt!). However, assuming that they’re right, when I want it cooler I can get the same result by either increasing the fan speed or lowering the thermostat.



Does anybody know which of these 2 options makes the most sense fuel-wise?



Thanks!


#2

It really depends on the car. AC is much more efficient then they were 10-20 years ago. And cars are more aerodynamic then they were 10-20 years ago. Some cars having the windows open puts much more drag on the vehicle then running the AC…Other vehicles it doesn’t.


#3

"Does anybody know which of these 2 options makes the most sense fuel-wise? "
There’s really very little difference.

Turning the thermostat to the coldest setting and then reducing fan speed is slightly better.
Turning the thermostat warmer does not reduce the cooling going on (unlike your home thermostat). Instead, it just starts adding in more and more heat from the engine.


#4

“it just starts adding in more and more heat from the engine.”

Never would have figured that out! Thanks!


#5

It depends upon your make and model car. Some have automatic climate control – set the temp and leave it there. Others are manual – select heat or A/C and adjust the fan speed. Read you owner’s manual. Neither one will make any measurable difference in mileage.

Twotone


#6

You would need the resources of a research lab to even try to measure the difference.

One idea that was presented to me in an AC seminar was that the slower the air flows across the evaporator the more heat it will give up. This flys in the face of what most here in AZ do when they get in their car that has been heated by our summer sun, which is fan to max, temp knob to minimum, I do realise you want to blow that hot air out of the car.


#7

Cars don’t really have “thermostats”. They have air blending valves that mix cold air from the A/C evaporator with warm air form the heater core.

Whether you use low or high fan speeds makes only a miniscule difference in power consumed by the fan; not really measurable.

Just set the temperature and fan speed to where you like it best; my wife prefers cold air with the fan on it slowest setting, so that’s what we do.