does anyone know how much driving with the ac on effects MPG. Sorry if there is a thread about this already i did a quick search and didnt see anything. I reset my mpg average last weekend and took a trip to cleveland. 500 miles round trip with the ac on and still got 39 MPG at around 70 mph. seems fairly insignificant but i was still curious if anyone knew actual numbers.

 

Myth Busters did a comparison with AC on and AC off (windows down) and found that the windows down had the most noticeable difference in MPG. They came to the conclusion that wind resistance had the most effect.

 

With the rain here in Norcal the last week I have had to run the defog/defrost (which turns in the AC) at lot more often. I have seen a 2 mpg decrease.....well,that and my lead rt foot...l

 

Myth buster did a test on highway. To see if window rolled down had better gas mileage than the a/c turned on with window up..

On highway, turn your a/c on and roll your window up if its hot.

On city driving, you will get better gas mileage with the windows rolled down.

i forgot exactly how fast they were going on the highway, maybe around 60ish.

 

Modern A/C compressors are pretty efficient. I would wager just having the extra electrical load from the blower and the 2nd rad fan would be just as bad the compressor alone.

With that said, as others have mentioned aero drag (increased engine load at the same rpm, therefore more fuel required) from the windows being open beyond a certain threshold can and eventually will outweigh the added engine load from the alternator and the compressor.

In town, windows down and vice versa on the highway.

 

I don't think the OP was asking AC vs windows down, he was just asking the effect of AC vs no AC (with windows up). I'm guessing 1-2 mpg.

 

You must have been driving on the only warm day we've had so far

 

Cincinnati to cleveland to see Adam corolla perform last weekend it was the wierd freak 70 something degree weekend

 

The AC cycles at lower temperatures so at 70 mph at 70 degrees with windows closed the AC is cycling. The OBC saying 39 is more like 35-36 actual mpg. The ultra gauge in my civic says at 70 mph, says I am getting 55 mpg but average 36-37 for the tank, doing the math. As the temps go up and higher loads the mpg will fall because the engine will use more HP about 10 percent average.

 

The rain has more to do with your 2 mpg decrease than the AC did.

 

The hotter it gets outside the lower the MPG to a point, because the compressor cycle is longer. Defrost in winter the comp. cycles a few seconds ever minute or so. In 105 temps w/ 90% RH the comp. will not cycle off and takes a harder hit on MPG. As I remember last summer I was seeing 3mpg loss from june - sept. on the atlantic coast (Va. Beach).
Big Mike

 

I rarely saw a difference with A/C on. I remember driving mainly on the freeway with A/C on and still got 38 mpg. Weather contributes to mpg the most. The colder it is outside the more my mpg gets lower. 32-34mpg in winter & 34-40mpg in summer with A/C. Windows rolled up on freeway. Windows down and A/C off in city.

 

Rain actually raises mpg on average. The lower barometric pressure acts like higher octane and no knock retard. The defroster cycles so it can hurt mpg but not as bad a 90 degree day when the A/C is constantly working.

 

Rain increases rolling resistance as well as aerodynamic drag. I've always noticed a drop in mpg in rain and so have others I have talked to.

 

Typically you drive slower anyways and less knock retard because I seen the fuel trims pull fuel which means better mpg. I agree about rolling resistance but also lower barometric pressure off sets that too.

 

Correct me if I'm wrong. I'm sure you guys will. But I thought in the GE-08-Present Honda Fits, the AC compressor was activated automatically if you set your controls to defrost?

So hot or cold, there is no way unless your windows are clear, to avoid not using the compressor.

Infact, I seem to remember a thread about de-activating, modifying this feature.

IMO the hit one takes using AC is usually well worth it. Either quickly defrosted windows, OR a cool cabin on a hot day is well worth a few mpg's to me...and that's all the hit I think it is...I haven't noticed much more impact than that.

 

My mpg hit due to A/C compressor kicking in, either during hot days (A/C) or cold season (defogger) is about 1 or 2 Mpg as shared by other posters.

However, IMO, I experience the greatest hit on mpg during city driving is directly related to how often I floor the gas pedal and did rabbit launch forward after getting green light at intersections.

Having learn my lesson, here's how I save gas for City driving for my AT which most people already knew and will not work for everybody if you need to reach your destination quickly:-

1) Gently step on gas when accelerating from stationery position - no jack rabbit starts
2) When going down the slope, remove foot from gas pedal and let the FIT roll down the hill
3) When climbing up the slope, I will try to not rev beyond 3K rev
4) The bottom line is the more gentle I am on the gas pedal, the better the mpg

The downside is that my journey will take longer due to my leisurely speed and I've tested the patience of many motorists tailgating behind me... well, the pay off for me is I am paying $35 per full tank of gas every 10 days... and I also save about $500 without the need to purchase a radar detector passport 9500ix or valentine or another $500 for radar jammer

 

^^ Ric,

I'd disagree on number 2. Removing your foot from the gas pedal will actually engage engine braking which means you lose potential momentum or even slow down. keeping your foot very lightly on the gas pedal will prevent engine braking in turn allowing you to maintain momentum while using almost no more gas (the instant mpg meter will still stay pegged on the far right) and you'll have that extra little bit of momentum at the bottom of the hill (where you usually proceed back up again - at least around here)

As for the AC and fuel economy, i lose about 2-3mpg due to the hills around here but usually we don't have a lot of need for it in the summer.

~SB

 

Same here. The hills near my house (for many miles) definitely bring down my mpg average. I'm still averaging 34-35 per tank on my 2011 Fit Sport but without all the hills near my house I'd probably average at least 1 or 2 mpg better, probably more.

 

Thanks SB for your tip. Will try out.

I am risking asking a dumb 101 question here....

The automatic engine braking you mentioned - is it peculiar to the FIT or engine braking is a universal phenomena for all AT vehicles.. all manufacturers? For MT, we just step on the clutch, change to neutral gear and let the vehicle roll down but not on AT? If that's the case, I should have bought MT instead of AT