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Does a car use more energy/gas when you have the cold Ac running full blast compared to the slowest speed?? Stupid question but I have heard diff answers from people Ive asked.

 
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short answer: yes, you use more energy/gas when you run your AC colder.


long answer:

any time the AC COMPRESSOR is running (your AC could be on and running, but the compressor not running) it will put strain on your engine...and as a result you get less power to the wheels and your MPG suffers.

with that said, when you have the AC on low (less cold) it will run your AC Compressor more seldom because it doesnt have to continually keep your cabin super cold.

if you run your AC on high (coldest) it will keep your AC compressor running more often and longer durations of time to keep your insides as cold as possible.

 
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He's right, but to add to that he the blower speed has nothing to do with it. He is talking about the temperature. For me I choose to get 1 or 2 mpg less to be comfortable. Not a big deal to me to get 10 or 20 more miles per tank

 
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the a/c compressor functions the same regardless of the fan speed. try for yourself and count how many times the compressor turns on in a minute for full fan and partial fan. There is no hi/lo for the ac on these cars, only for the blower motor which is completely unrelated other than it blows more or less air over the evaporator. the only time it may matter is when the temps are close to freezing and the a/c may kick off if it senses the evaporator is too cold (would happen more on a lower fan speed). otherwise there is no temp sensing function. even if the temp dial is set to hot, the ac would still work the same amount. you guys give this car way too much credit. maybe the hybrid civic would be different because it has a temp probe and an auto setting that you can set like a thermostat.

but to answer the op's question, yes the fan speed does affect overall power usage (and in turn, mpg). Each one tick of the fan uses 55w of power, the equivalent of turning on one halogen headlight. If you have it on two ticks from off, its like you turned on your headlights. two more ticks, like having 4 headlights on. that does add up and combined with the already heavy strain of the hi amp cooling fans on the radiator, can add up quickly for this poor car with tiny alternator. it is best not to use ac unless you absolutely have to.

 
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Stand corrected, but still same 10 or 20 more miles per tank applies to me still

 
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I hear having the AC on puts extra wear on your engine and wears it out faster.

 
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so do the axles, ps pump, alternator, and anything else connected to the engine.

 
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actually, the blower speed IS in fact apart of the equation.

the faster the blower blows, the faster it draws cold air from the evaporator core (i think thats what you call it)...and therefore the ac compressor is be turned on for a longer period of time or more often to keep the same cold temp.


so for example if you set your temp control to coldest, and you set your fan speed on low, your compressor will turn on less often (than if you put the fan speed to high) because there is a lower flow rate of air passing the evaporator to turn the air cold; and therefore the evaporator can stay cold a bit longer (within a set range of temp) before the AC compressor kicks back on.

at least thats what seen with my car.

 
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What the hell man lol. Maybe I should trade it in if I shouldnt use the AC unless it's steaming outside.