MPG w/ AC Question

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.

 

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.

 

Have you checked the oil level on the dipstick yet?

 

You may also want to check your 12v battery.

 

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!

 

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.
.

 

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

 

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.

 

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.

 

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.

 

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.

 

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.

 

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.

 

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.

 

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.