MPG seems impacted by heat and cold

Correct. Without climate control, fuel economy improves with temperature. Climate control in summer gives a small hit with highway driving, a bigger hit with city driving, and potentially huge with stop-and-go conditions.

 

Mine last tank was 38MPG think because of all the short trips, everything is 1/2 mile or less

 

A/C (both heating and cooling) takes energy to use, and that energy is generated with gas...

mpg also takes a hit from air density when cold, and a boost from hot air (less dense).

 

You may want to reconsider your use of A/C on those short trips, unless your car interior already is cooled. Also, when using A/C you may want to set the temp higher (say 75 or 76) and limit the fan speed instead of using Auto. Each of these steps saves energy! The first five minutes also is highly critical, so you could consider limiting the fan a bit more for those few minutes.

And that humid Carolina air is even less dense. This helps, to a point: when the dew point is high enough to approach the ambient temp, the condensation formed on the car's surface can start interfering with those "gains" from lower density air!

 

Adding to the above, the battery is "happy" at roughly the same temperatures as the occupants. In mid winter the car is inclined to charge the HV battery to double green bars at times until it is up to temperature. There are some specific temp threshholds it targets (if I correctly recall what others reported.)

At some point the battery can get too warm and require cooling. I've not noticed this happening yet and don't know how the battery cooling fan sounds. But apparently it could also be counterproductive to try to be Spartan and soldier along at 110 F in the cabin.

The battery temperature mileage effect is one that conventional vehicles don't experience. It is pretty noticeable at times when the temps are at/below the teens.

 

looks like our 100+ weather has died for awhile but I still drive with the AC off. Do you get some sort of warning if the battery is too hot?

 

No direct warning. You might hear the battery fan cut on, though I've never heard it either. If you're really attuned to the car's operational nuances, you might notice a change like I describe here.

Pardon the cliché, but prevention is the best medicine. The battery can easily get to 100F+ after baking in the sun all day, and its large thermal mass makes it slow to cool (and warm in the winter). I have tinted windows, I use a sunshade, I crack the windows, and IMHO most important, I park in the shade when possible.

 

It has been reported that the battery fan sounds like a turbofan on a jet. I believe the car will turn on the AC automatically to cool the cabin to cool the battery. Yes, the car doesn't like too cold and too cold. It seems to run best in the 60's.

 

The temperature extremes at the battery also limit or prevent transfers to/from the battery as I understand it. (If someone here has better/more detailed information the please correct this.) When the battery control system is unable/unwilling to allow the HV battery to be used as intended, your mileage will tend to suffer as the engine will be on more. That is what I meant about it being counterproductive not to use the AC when it is getting very toasty in the cabin.

 

Undoubtedly. That seemed to be what was happening as I described in the link previously. And in cold weather, a battery temperature of 35F seems to be near the magic number below which the car is reluctant to use it, as some of us describe here. I've seen that, BTW, only with extremely low ambient temps, like single digits (F).

To the OP, to bring this back a little closer to your focus: These are indeed extremes that we're talking here, not likely seen except on the hottest and coldest days of the year. In routine summer or winter driving in your climate, the battery will behave as it does in more moderate weather.

 

I has been up to 104 and I have yet to hear the fan run.

 

My best mileage so far is with the AC off and windows cracked. Up to 58+average best so far but not every tank/ with 44-47 PSI and 0W30 synthetic oil.

 

After the 3rd tank of gas on the new synthetic oil my MPG started to go up and I noticed a smoother stronger engine.

 

If you want to hear the fan run, stick your ear right next to it; twice I've heard the fan from the drivers seat, as it came on at what was obviously full power; however, recently I've been in the back more often and, sitting in the right rear seat, I can nearly always hear the fan, but its pretty quiet.

 

I don't know what happened, but I'm getting only 45mpg, whereas I was getting 48 or better. It could be the heat here in Austin, TX... it's been over 100 for weeks now. I used fuel injector cleaner for a couple of tanks of gas thinking that might help but no. Maybe just dealing with 'more heat' in the car by setting the A/C higher/cracking the windows while driving at slow speeds. I'll check all the fluid levels and up the tire pressure some next time I refuel, and 'drive like an old lady' and see if I can't get the mileage back up there.
I still think the best ambient temp is in the low 70's...

 

With AC aside, there is a difference in mpg at temp extremes. We have a 2007 TCH near Chicago. We get 38-40 in summer (record tank was 42.5 so far). In the dead of winter we average about 34 mpg, no way to get 40 mpg when it's consistenly below freezing (as happens here). Ambient temp is a significant factor from our experience, I assume it's the same for the same reasons with the Prius.

 

40 in the summer? That's horrible,, 35 in the winter sucks as well. How do you drive? do you block the grill at all? We've got ~40k on our 07 Very seldom is there a tank under 50,, most ~52 mix of city and country. We just got back from a 7000 mile coast to coast and then some trip. 3 tanks under 50,, one with terrible headwinds of 42, one with great tail winds of 63.

This driving 65-75 the whole way,

If you are only getting 40 in the summer and less in the winter I would look at your driving style,, too much right foot.

Icarus

 

If its hot when I get in the car I open the right side rear window (as well as both front windows) fully for the first 5 minutes of driving. You would be amazed on how much that helps MPG.

 

I'm in ATX, too and it has been hot as hell lately.

When you use the AC in your car, it takes power. That power has to come from somewhere. That somewhere is the gas that your engine burns. Not directly like belt-driven units in standard cars, but through electrical generation.

Without the AC on, electrical power generated while cruising and while braking are mostly only used for propulsion (with a small portion being used for starting the ICE and running all the peripheral systems). Turn on the AC, and there's a large load on the electrical system. The AC will SUCK SOME JUICE!! Especially in hot weather.


Here's some tips for those of us in the south:

• Get your windows tinted. Dark.
• Use a sun shade in the front windshield. All the time. Every time. Use it RELIGIOUSLY.
• Park in shade where you can.
• Crack a couple windows open so hot air can vent out.

When starting a trip:

• Roll down the windows for the first few minutes
• Set the AC to fresh air for the first few minutes
• When the cabin air has been flushed out and the AC is getting cool, roll up the windows, but leave the AC on fresh
• After a few minutes when the cabin starts cooling, switch the AC to recirculate
• Set the temperature as high as you can bear it comfortably to reduce the power used by the AC

I'm SO glad I can park my car in the garage at home, and in a parking garage at work. Keeping it out of the sun works wonders in mitigating heat.