Fuel economy in 100+ degree weather.

I use the standard engineering definition. Of course you are free to use any definition you want.

 

I use the standard calculations that determine horsepower and 1/4 mile projections as this is where I spent most of my adult life until I bought the Prius. lol

I understand you are thinking in terms of static conditions but since we do not live in a static world I am looking at real world variable conditions.

 

I am averaging between 54.2 & 57.3mpg per tank on the tanks from June & July which cover the 100F temps.

 

Clinton AR 104 today, got 55 mpg at an average speed of 55 on the way home. 8100 miles on the clock. Been doing a consistant 51 all summer as per the MFD. AC at 75 auto, tires 46/44.

 

AND at least every two weeks.

 

High dewpoint means more water molecules in the air. Relative humidity is affected by dewpoint and temperature, so it's better to look at dewpoint and temp separately.

Since, water has a molecular weight of 18 vs air at about 29, more vapor means lower average MW and lower density (at the same barometric pressure and temperature). But, it's a very small change. With lower air density, drag (at constant speed) goes down. Comfortable air is about 1% water, and very wet air (gulf coast air) is about 2.5% water...so over a wide range of dewpoints, 1.5% of the molecules go from 29 to 18, so the change in density (from dewpoint) is less than 1%.

Higher temperatures also lower air density, and rolling resistance. Tires' pressures go up with temperature, and rubber viscosity goes down...both help.

So, as far as drag goes, directionally, hot/humid is better.

Water vapor is a diluent in air, so while engine max (full throttle) power goes down, at part throttle, more manifold pressure is needed for a given power. So, water vapor acts just like EGR, reducing pumping losses. Same for temperature. A volume of air has less O2, so more manifold pressure is needed to deliver the same amount of O2...lowering pumping losses at part throttle...and most fuel is consumed at part throttle.

The big (huge) negative is AC load. Humidity needs to be condensed in order to cool the air, and this heat of condensation has to be pumped out. Heat load is higher too, meaning even more cooling capacity. With the hotter air across the condenser , the compressor has to pump the freon to a higher pressure, which takes more work. It's like a tripple whammy to the AC...more water, more heat load, higher high-side pressure.

The way I drive the car, it seems to be about a wash. I'm still averaging an indicated 53 (with 80F in the morning and 104F in the evening)..about the same as I was back in the spring.

 

i live in arizona where the temperature averages 110 degrees everyday.

my mileage is displayed below.

 

I’ve driven 50 miles on the current tank and am getting a miserable 32 mpg. Under similar driving conditions in cooler weather I would expect that to be up around 50.

My results appear to be much worse than others in this thread but this can be explained by my driving conditions. I live in southern Arizona where the average high for the next week is 112. The sun is also very intense and we very rarely see a single cloud to provide even momentary relief. I usually have the AC set somewhere between 74 and 78. My typical drive is usually less than 30 minutes and with the sun load it will run full blast the entire time regardless of where it is set. Combine that with the horrendous traffic flow in this town where I spend almost as much time stopped as I do moving, and of course I take a tremendous hit from the heat.

 

Well, my 2010 Prius III has 5100 miles on it here in Orlando. I bought it in Dec. last year, and was getting 48mpg local (50 indicated) during the winter months. Once it hit 90+°, mileage fell to 40mpg actual, 42 indicated. We keep the a/c at 71°. Of course, we can only buy E10 gasoline in Florida, so that accounts for 15% of the loss. Took it to the dealer for it's first check and expressed my disappointment. Dealer said nothing can be done as it is all under the control of the computers.
Ed

 

I'm not sure I've ever seen anything that states a 15% loss when using E10 fuel. That seems like a rather drastic drop compared to what everyone else using that fuel experiences. I think your loss is associated with your heavy use of A/C in stop and go traffic and/or short trips.

 

Good to see this topic! I was surprised to see my mileage start dropping today, but it's 111° and rising here in Dallas and I was suspecting it was due to intense heat tho I've never had this happen before in slightly lesser heat (ca. 100- 108°). Good to see that I'm not one of the few. Didn't think it necessary to renew this topic under the first gen Prius area, but mine is an '02 with just under 92,000 - dropped from 55.8 to 55.6 today after continuously rising every few days.
(forgot to mention that this is without AC use)
Really too hot to be out in this anyway!

best to all of you,

Rod

 

I live in Maryland, 100F in my area and my MPG has dropped some.Highway driving seems to be better for milage in the hot weather.

 

Living well north of you, it's not quite so bad. We've had mid to upper nineties and heat indices around 105. I guess I'm blessed in that the heat doesn't bother me much so I always turn the A/C off in the morning. Also, I use ECO which does decrease the A/C's efficiency but it's good nuff. I turn it off at stop lights (stops the ICE). I learned this from using the heat in the winter.
Good luck, and maybe a few tricks will help. You have a vast source of knowledge in this group.

 

if you don't turn on the AC, you can get pretty good millage

 

71 degrees?? Brrr. We run ours at 76 (AUTO) in Ocala and in ECO mode and are approaching 50 MPG on our first tank of fuel.