MPG w/ AC Question

Discussion in 'Gen 2 Prius Main Forum' started by ack714, Jul 3, 2011.

  1. ack714

    ack714 Junior Member

    Joined:
    Mar 5, 2011
    42
    1
    0
    Location:
    virginia
    Vehicle:
    2008 Prius
    Hi All... Happy 4th,

    I have a 2008 with 88000 miles. Up until recently my MPG estimate given under info has been between 42 and 46 MPG. Recently it is showing it as 32MPG. There have been only 2 changes. I got the oil changed and we have been running the AC full time with the summer weather. Can running the AC actually suck up that much gas? We havent been putting as much mileage on the vehicle either with school being out.Probably 500 miles less per week.
    BTW we love the car. No problems.
     
  2. douglasjre

    douglasjre Active Member

    Joined:
    Sep 13, 2008
    609
    247
    0
    Location:
    Orlando, FL
    Vehicle:
    2010 Prius
    Model:
    ----USA----
    No and no. Reset your trip meters, and on screen fuel eco meter. Compare your calculation between fill up with fresh readings on screensomething else is going on.
     
  3. Danny Hamilton

    Danny Hamilton Active Member

    Joined:
    Apr 30, 2007
    926
    92
    0
    Location:
    Greater Chicagoland Area
    Vehicle:
    2007 Prius
    Model:
    N/A
    Increased A/C usage can certainly increase fuel usage, how much depends on a variety of variables such as outdoor temperature, how sunny it is, and what temperature you have the A/C set at. How much it affects MPG also depends on average speed (Since the affect of A/C on fuel use is time dependent rather than distance dependent it will hit MPG more significantly with slower average speed).

    Also if you have been driving in precipitation (rain/snow) more often now, that also can reduce MPG significanlty.

    Additionally long trips give better MPG than short ones, so if you are making more shorter drives, and less longer ones you'll see your MPG go down.

    There can be other small factors (such as oil overfilled during the oil change or tire pressure is lower now for some reason), but the three listed above are the most likely typical culprits.

    I think the difference between 42 MPG and 32 MPG works out to about 5.7 teaspoons per mile, but it is possible I made a mistake in my math there.
     
  4. uart

    uart Senior Member

    Joined:
    Sep 7, 2009
    4,215
    1,180
    0
    Location:
    Australia
    Vehicle:
    2005 Prius
    Have you checked the oil level on the dipstick yet?
     
  5. car78412

    car78412 Member

    Joined:
    Jul 18, 2010
    279
    31
    3
    Location:
    Hudson Valley, NY
    Vehicle:
    2010 Prius
    Model:
    IV
    You may also want to check your 12v battery.
     
  6. bc104

    bc104 Junior Member

    Joined:
    Mar 6, 2007
    57
    2
    0
    Location:
    New York
    Vehicle:
    2007 Prius
    It's the AC. I have an '07 and I don't use the AC that much. Maybe on the highway. The AC uses a lot of electric (it's an electric compressor and doesn't run off the engine) and therefore the engine is spending a lot of it's energy charging the battery. If you are driving down the road, it's like having a trailer attached. If you're in traffic, the battery bars will be all the way down and every time you take your foot off the brake the engine will start.

    In general it's just mien the car and I don't use the AC. If the AC is on while I'm on the highway, I'll shut it off in traffic. It's just me, so it's OK. if the family is in there, well, that's another story! :eek:
     
  7. john1701a

    john1701a Prius Guru

    Joined:
    Jan 6, 2004
    11,948
    4,666
    57
    Location:
    Minnesota
    Vehicle:
    2017 Prius Prime
    Model:
    Prime Advanced
    It's not the A/C alone to blame. That system is surprisingly efficient. Overfilled oil, possibly the wrong oil type, squishy tires, and an noticed driving pattern change (with school out now) are very likely contributing to the drop.
    .
     
  8. edthefox5

    edthefox5 Senior Member

    Joined:
    Jul 25, 2007
    8,411
    3,833
    0
    Location:
    Clearwater, Florida
    Vehicle:
    2007 Prius
    Model:
    Two
    Personally constant & heavy AC use has not impacted me at all. Even in stop & go when it gets to one bar it recharges very fast. Within 2 blocks.
    I regularly get 48 to 50 mpg's and the ac is never off year round in my car. Never ever off. Its brutally hot down here.
    Plus you do not want to drive around without the ac on as your letting moist air into the cabin. There's thousands of electrical connections in the cabin especially the hybrid battery power connections and they will corrode anyway.
    You don't want to hasten that process along by filling the cabin
    with moist unconditioned air. Plus do all you can to keep the hybrid battery in the back cool.

    And it doesn't take much to impact mileage on this car. I just had extremely good luck changing the Inverter coolant.

    Other top picks:
    Bad small 12 volt in trunk ( if you have ever had to jumpstart the car just once it
    prob. needs to be replaced)
    Clean air filter.
    No oil overfill.
    Clean the throttle body.
    Clean the maf.
    Inflate tires to 48-46
     
  9. ack714

    ack714 Junior Member

    Joined:
    Mar 5, 2011
    42
    1
    0
    Location:
    virginia
    Vehicle:
    2008 Prius
    Thanks yall, I mentioned the oil change because when my mileage changed i checked the tire pressure and found that they lowered my tires to 30 lbs . I know they removed my filters and put them back because they were clean. I set my tires to 1 lb under maximum in the front and 3 lbs in the rear. I really don't know Wth they were thinking setting my tires at 30. I'll have to check that the filters were put back properly, and if the oil is too high..thanks
     
  10. ack714

    ack714 Junior Member

    Joined:
    Mar 5, 2011
    42
    1
    0
    Location:
    virginia
    Vehicle:
    2008 Prius
    When I checked the oil, it measures just above the full line on the dip stick by around 1/4 inch. I guess they put a full 4 quarts in plus whatever they didnt let drain. The filters look good. After a week on the increased tire pressure the mileage is up to 35MPG. My tank is emptying so I can check the MPG myself in a day or 2.

    thanks
     
  11. uart

    uart Senior Member

    Joined:
    Sep 7, 2009
    4,215
    1,180
    0
    Location:
    Australia
    Vehicle:
    2005 Prius
    Filters? I hope you're talking about the air and the cabin filters and not the oil filter! They definitely shouldn't be re-using old oil filters.

    As for the oil being 1/4" over filled. While that level is probably not too serious, it is getting up around the point where it can start giving problems. You probably should drain a bit out and get that back to the top mark or just under.
     
  12. flareak

    flareak Fleet Captain

    Joined:
    Mar 9, 2004
    1,016
    20
    0
    Location:
    St Louis, MO
    Vehicle:
    2005 Prius
    Model:
    N/A
    If you're blasting A/C when the car is at stop or low speeds like in a traffic jam.. you can actually get the big battery to drain within.. i'd say 10 or so minutes. Then you're pretty much using the ICE all the time to move the car and it'll drop your MPG significantly when your battery is in the pink.
     
  13. itonarely

    itonarely Junior Member

    Joined:
    May 3, 2011
    36
    12
    0
    Location:
    Miami
    Vehicle:
    2016 Prius
    Model:
    Two
    Driving in Miami, my a/c is on as soon as the car is (set to 68 on the lowest or second-to-lowest fan setting).

    We haven't hit "summer" yet but I have been driving my '11 for almost three months now and my mpg has consistently stayed between 49 and 50 mpg (according to the mfd, a little less by my own calculations).

    Perhaps if I turn off the a/c I will get better mileage but that isn't going to happen.

    Still, I don't think the a/c should affect your mileage that much. I would also take a look at your daily driving routine. I have noticed that changes in your trips can make a big difference in your mpg. Good luck.
     
  14. itonarely

    itonarely Junior Member

    Joined:
    May 3, 2011
    36
    12
    0
    Location:
    Miami
    Vehicle:
    2016 Prius
    Model:
    Two
    Driving in Miami, my a/c is on as soon as the car is (set to 68 on the lowest or second-to-lowest fan setting).

    We haven't hit "summer" yet but I have been driving my '11 for almost three months now and my mpg has consistently stayed between 49 and 50 mpg (according to the mfd, a little less by my own calculations).

    Perhaps if I turn off the a/c I will get better mileage but that isn't going to happen.

    Still, I don't think the a/c should affect your mileage that much. I would also take a look at your daily driving routine. I have noticed that changes in your trips can make a big difference in your mpg. Good luck.
     
  15. fgoodyear

    fgoodyear New Member

    Joined:
    Aug 28, 2007
    187
    10
    0
    Location:
    Tillson, New York
    Vehicle:
    2007 Prius
    Had mine since June of "07. A/C has never affected gas mileage. In fact, I get better mileage in the summer than winter because the a/c is electric instead of on a belt ran by the engine. The tire pressure shouldn't be all of the problem. They always adjusted mine down and it affected it a little. Like you were told, reset and check your mileage again. If not, at that mileage, mine has 90k, check the 12 v battery. I have heard that when it starts going, it can do some weird things.
     
  16. ack714

    ack714 Junior Member

    Joined:
    Mar 5, 2011
    42
    1
    0
    Location:
    virginia
    Vehicle:
    2008 Prius
    I did reset my trip Meter. I got 80 miles less with this tank.
    When I filled up yesterday I reset them again.we'll see what happens. It sounds like some owners are saying AC can affect MPG and some are saying it doesn't. I'm just going to drive it.I appreciate the feed back from all.
     
  17. prius4ed

    prius4ed Member

    Joined:
    May 13, 2008
    120
    4
    0
    Location:
    Irvine, CA
    Vehicle:
    2004 Prius
    Again, the VISCOSITY (eg, 5w30) of motor oil can make a difference (don't know how much, but I heard it was significant). So can overfilling the oil above the full mark. What is the suggested tire pressure for our cars? I always put mine at higher pressure, but stay below tire max pressure rating of course.
     
  18. Danny Hamilton

    Danny Hamilton Active Member

    Joined:
    Apr 30, 2007
    926
    92
    0
    Location:
    Greater Chicagoland Area
    Vehicle:
    2007 Prius
    Model:
    N/A
    The A/C is electric. Since the Prius does not get plugged in, all electricity comes from burning gasoline. Therefore since the A/C causes an increase in the volume of gasoline burned (the denominator of the ratio) without causing any increase in the distance traveled (the numerator of the ratio) it is clear that the A/C must have an effect on MPG. The only question is how much of an effect it has. This is highly variable. At continuous high speeds of travel the distance traveled in a given amount of time is large, so small changes in the volume over the same amount of time results in small changes to MPG. At slow speeds of travel (or at a complete stop) the distance traveled in a given amount of time is small, so small changes in volume have a larger effect on MPG.

    Example:
    I average 20 MPH for 1 hour and burn 0.33 gallons of gasoline.
    20 MPH X 1 hour = 20 miles
    20 miles / 0.33 gallons = 60.60 MPG

    You average 80 MPH for 1 hour and burn 1.6 gallons of gasoline.
    80 MPH X 1 hour = 80 miles
    80 miles / 1.6 gallons = 50 MPG

    Now we both turn on the A/C and drive the exact same trip. If the settings we use on the A/C required 0.2 gallons per hour to keep the cabin at the set temperature. . .

    I use 0.33 gallons for locomotion + 0.2 gallons for A/C = 0.53 gallons total
    I now get 20 miles / 0.53 gallons = 37.74 MPG
    Ive dropped from 60.60 to 37.74 by turning on the A/C. A change of 22.86 MPG!

    You use 1.6 gallons for locomotion + 0.2 gallons for A/C = 1.8 gallons
    You now get 80 miles / 1.8 gallons = 44.4 MPG
    You've only dropped from 50 MPG to 44.4 MPG, a change of only 5.6 MPG!

    I've gone from way better than you to way worse than you and yet we are both only burning an extra 0.2 gallons per hour by turning on the A/C.

    The amount of fuel needed per hour to maintain the desired cabin temperature will depend on many variables. A few are: outside temperature, temperature set on the A/C, amount of cloud cover/shade, wind speed, color of vehicle and interior, time of day, your latitude on the earth.

    So someone who drives a white car with a white cloth interior a long distance at 80 MPH on a tree lined expressway in Maine on a cloudy morning with a tail wind will find the A/C has minimal effect on MPG. Meanwhile someone who drives a black car with black leather interior a short distance at 20 MPH on a unsheltered road in Brazil on a clear afternoon with a head wind will find the A/C has a huge effect on MPG, even if the temperature is 95 degrees Fahrenheit in both locations and the interior temperature is set identically.

    All this to say, yes, A/C absolutely, positively, certainly, definitely has an effect on MPG, but that it may effect one person's MPG almost imperceptibly while affecting another person's MPG significantly. You may very well fall into the "significantly" category now that summer is here and your driving habits/routine have changed.
     
    3 people like this.
  19. ack714

    ack714 Junior Member

    Joined:
    Mar 5, 2011
    42
    1
    0
    Location:
    virginia
    Vehicle:
    2008 Prius
    thanks for the time Danny.I appreciate yours, and all the well thought out responses to my question. Y'all have been very helpful
     
  20. sktn77a

    sktn77a Member

    Joined:
    Apr 28, 2007
    290
    12
    0
    Location:
    NC
    Vehicle:
    2005 Prius
    Providing you aren't using 80Wt gear oil (!), that's not your problem. A/C reduces the MPG by most small ICE-only econoboxes by 2-3mpg. I would imagine it would reduce the MPG in the Prius by more than that but how much more, I don't know.
     
Loading...