AC commits pre-meditated murder on MPG

Hot as heck last week, so AC on bust. And busting up my MPG hard. Last week I was at 50 for a tank (no AC), this current tank of 250 miles I'm at 46 and dropping!

Seems to hurt it 4-5 mpg, unless I want to blame the new trailer hitch I put in last week (I don't). No, I don't use ECO mode Yes, I know AC hurts all cars' mileage!

Question 1 How much does heavy AC use cut YOUR mileage?

Question 2 Though the compressor is on in any case, how much better would mileage be using AC on low fan setting instead of high?

With my 2008 Prius the AC cuts my mpg about 1 mile per gallon. It's not easy to even tell to be honest. I'll use the AC quite a bit when it's over 85 degrees but I also track my mpg so...

I doubt though I ever use the AC as heavy as you did, as it's very loud when it's turned on full.

On my 26 mile daily commute the AC has very little impact at highway speeds. Maybe 1 mpg. Once the cabin cools down and the AC fan and compressor slow down I can actually start gaining mpg back. This is in 95 F and 95 % humidity lately.

Now, around town on short hops, yes it will definitely make a hit on mpg. The AC will be running wide open most of the time trying to cool the cabin down.

how much do you get hit?

I swear it wasn't this bad last summer, I'm now down to almost 45 just on the way home, but I have had it on bust the whole time.

Crack a window and be happy at lower speeds.

What's the outside temp? What do you have the inside temp set at? I assume you have it set on recirculate, blowing through the top vents only? I think there is an ECON setting on the AC, if so are you using it?

I drove home from Boston today, outside temps 86-88F and humid. I was almost ready to turn on the AC but just managed it with windows cracked and fan on.

A week or two ago under roughly 90F conditions I did fire up the AC for about half the trip because I had the dog in the back (no dog today). I ran it at 75F, and even just the dehumidification helped a lot. No huge effect on MPG - got the usual 65-70 mpg on the HSI. Admittedly not a very quantitative test though.

you said the hitch doesn't effect mpg but i tend to usually disagree with that unless it's a really good hidden hitch. with just the 2 inches of hitch bar hanging below the car, i saw a mph hit.

unless you're cruising on the highway, the drain you explain sounds rather normal.

people used to make fun of me when i first had my car... a total wtf cuz they were like " 40mpg... 45mpg.. what's the difference" while i know it adds up in the end.. really what is the difference. i have a trailer and i get about 30 to 40mph (usually 30 while towing)... no other car can do that.. just like basically no other car gets your gas mileage with all that you get... so i ask myself, what's the point? yes you can get better gas mileage yet you already do get better gas mileage.

it could be worse.. you could turn off the a/c to save the electricity but then your battery will take a hit from too much warm air... how does this directly effect, we don't know... but everyone assumes it reduces the life of the battery.

this is why people say " just drive it"... cuz it's already a leader, and any mpg you get is better than what you owned before (right?)

Outside temp around 82 or so. I find once it's above around mid 70's I want AC on. I am hot natured it seems and going without AC is like how the savages used to drive in the middle ages.

I don't think it's on recirculate. I always have the temp set to LOW, never use auto mode, and control the temperature manually by the amount of fan speed. That's just how I like to roll

I will probably get some thin plastic sheets and shape a wedge and tape to the front of the hitch just for the heck of it, though I don't know how much it impacts things. My wife always said that idling the car, even for short periods with AC on was murdering gas. I guess I Just didn't believe it until now!

I haven't had a full tank or long trip that I used a/c I know it does impact quite a bit along with heavy rain on my moderate speed (not much highway normally since a lot of driving is in Chicago). I have noticed probably 4 MPG difference and I do drive in eco mode. I usually use A/C when my wife is in car (she is all of 100 lbs so not much extra weight there....).

I usually only use a/c when temps in mid upper 80's or higher.

I get 47 MPG fall and spring, and 42 MPG summer. MS features high heat and high humidity, and I am not a local.

I get 0 mpg when I leave the AC on in the parking lot. But it's worth it when I return to a cold car instead of a blast furnace.

Here in Austin, it's been at least 100F or more since end of May. Only a few days we're below 100F. Since then, avg is about 42mpg but the last tank was 40mpg. Prior to that, it was a steady 48mpg. No way I can drive around town with the windows rolled down.

You can probably save by setting it to recirculate. Otherwise you are constantly cooling hot outside air rather than re-cooling somewhat cooler inside air.

With the temp set to LOW, the AC unit is constantly running at max cooling to try to get the interior as cool as possible. If you set the temp to some temperature, say 72 degrees, it will try to regulate to that temp instead. I think the Prius has a variable AC unit; if not it may simply turn itself on and off as needed. Either way uses less power. And just the dehumidification when the air passes thru the AC will make the air feel more comfortable.

Those two things should reduce the electrical usage by the AC unit and so help decrease the mpg hit of AC.

It has been over 90 for about 6 weeks in GA and I use the AC every day. I don't get great gas mileage as it is but I don't see much of a difference between on and off either.

I set at 72 (not on auto) and fan speed 3, pull from outside with driver window half down and opposite rear window down until I can feel cool air coming from the vents. I then run on outside until it starts to feel a little cooler in the car, then I switch to recirc.

I have been revamping my driving habits and went from 45.5 to 45.8 (tank average) on my 11mi trip home with the AC on and my 11mi trip average was over 50.

My question: If this country expressed fuel economy as gallons/100 miles, instead of MPG, would most of these complaints vanish?

I think so.

I'm currently getting hit for about 4.5mpg with my A/C on. It's been low 100's where I live with humidity in the 60's. My main drive is only about 9 miles each way though, so there's not a lot of time for the A/C to be on lower speeds.

I was in Las Vegas, NV for the weekend. High temperature was between 105 to 110F. I got about 40MPG, too (maybe slightly lower), but was running AC all the time, with temperature setting at about 69F, fan varied but relatively high, recirculate, and in ECO mode. There was no way I'd just roll down the windows (I'm too old for that).

Driving to and from Las Vegas, I did a little better at about 42MPG. Temperatures were lower, between 80-100F. Obviously, speed was higher and varied between 70-80MPH. Don't know how much difference that made.

As an aside, I was at the pool in Las Vegas for about 15 minutes and got sunburned. Way too hot for me.

Mike

No. That doesn't even make sense. It's representing the same thing (~10% worse fuel economy) regardless.

Seems like a lot of people are noticing similar hits, then.

I have a gen II but I find that AC on i'll drop to 40-42. AC off completely is around 46-48. Right now I do an AC off during speeds under 30mph, and AC on for highway and I'm currently at 46.6.

But I set my AC at 77 degrees and it's always 90+ outside..

You would probably do better with A/C Auto mode, where the Prius has the option of balancing fan speed and recirculation with the proper compressor setting.

Tom