Outside Temperature and MPG relationship

I'm in agreement with Evan 100%. On my tanks that are near 60 MPG the temps in the morning are mid 70s and in the afternoon are mid to upper 90s.

Sitting in traffic without moving with the AC running will kill the mileage. I often roll down the windows if I'm sitting at long lights. Once I'm moving I turn on the AC and roll up the windows. There are limits to this strategy based on comfort. Personally, I'm uncomfortable with watching my mileage vanish while sitting at a light.

 

I have lots of hills and the temp varies wildly here in the desert. Temps go from 30 deg in the morning to 105 in the afternoon (this psat week). So each day I get each extreme (although the last week it has only gotten to mid 60s in the morning). My fuel economy has pretty much stayed steady. Only about a 1-2 mpg hit for A/C or heater use. I travel the spd limit about 10 mi at 35 mph, 15 mi at 60 mph, then 5 mi at 25 mph. Lots of hills and windows open the first and last leg of the trip just in the mornings. I do tend to coast a lot and use cruise control unless I go below 24 mph. If no one is behind me I drive the spd of the terrain (between 2-10 mph slower than posted and up to 5-10 mph faster due to hills or stops less than 100 yds apart).

 

Actually I seem to notice this temp/mpg difference also. I think extremely warm temps (above 90) seem to bring down the mileage. We had a cool snap (only 79 yesterday) and my mileage improved about 5mpg. Don't think I was driving any different than I normally do, and I was in the same area as I usually travel.

I've only had the car a couple months, but I've noticed the mileage seems to suffer as it is getting into the 90's. My worst mileage to date was on a day in the low 100's. Of course I was running the air...no way around that. I'm not going to cook to get better mpg. 8)

I think it is necessary to keep the car cool to keep the battery cool? I may be mistaken on that point, but somehow got that impression. Is there any way to tell if the car/battery is overheating? I guess I should read the manual more carefully, but I don't see any gauge. I do remember that I'm not suppose to cover that vent in the back, so I figure it draws in cool air from there.

 

I'm in the Boston area too, and yet I find warm weather really helps mileage. Hot weather here so far seems to help more than the drain of using AC.

My commute is mostly at 30-40 mph on back routes, or 55 to 60 max on toute 128.

At say 50 degrees I might expect a commute at 60 plus mpg the back route, or 52 on 128.
At 80 degrees I might get over 70 mpg the back routes and over 55 on the highway.

Driving highway like if I go up 93 or on 495, I lose maybe 3-4 mpg due to the higher speeds.
But still warmer is better.

Someone had curves here I think, or maybe it was yahoo. Anyway from looking at them (they were just drag vs temperature and speed data) I noted that driving 55 mph in cold winter weather was about the same mpg wise as driving like 70 mph in hot weather.

My current commute is only12 miles each way. I have many trips less than that to shop.
My mileage dropped to low 40's in winter. For the last 4 tanks I have averaged over 51 mpg.

 

I can testify that sitting in traffic in 90 degree weather really kills the gas milage. I had a tank average of 55 last month and the temp went to the 90's. I thought I would clean up and get my first 60 mpg tank but for what ever reason the traffic got really awful, like waiting for the light to change 3 times before you even get to the intersection. No accidents just everyone going the same direction at once and my milage went down and down. Also around here even if the highs are in the 90's the lows will be in the low 50's so the morning commute can be a bit lower. Next time I am not going to drive the shortest rout but go around and be able to move!

 

well temps in the morning have reached 86 deg and afternoon are now 105+ so I am using the auto A/C more now which has droppd the MFD about 4 MPG.

 

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What temperature do you set the AC for? If you are setting it to something like 72 or below on a sunny 95 degree day, then that's what's dropping your milage.

When the AC is running full out, it really sucks down the battery charge. That causes the engine to run more and for it to put more of it's output into charging the battery.

I have found that on really hot days like that, setting the AC to 78 is sufficent. The AC doesn't run full out like that and after driving for a few minutes, the Auto/AC sets the fan speed to medium low. With the cool air from the vents aimed right at me, it feels just fine.

For cooler days (mid 80s), I will often set the temperature between 74 and 76.

With these settings, it doesn't seem to put too much of a dent in my mpg.

 

That's my experience, especially coming home where the cabin is really hot. Last week, the temps even at night wouldn't go much lower than 80, and so even my moring, late evening trips had more of an MPG hit than mornings this week. I currently have my temp set to 75.

 

Here in phoenix the temps have reached 110 to 115 and with the added AC usage, this has dropped my mileage below 40 for the first time . That's ok though: it still beats the truck's whopping 13 mpg!

 

I saw a cool spreadsheet in the Yahoo Prius-2G forum, which laid out expected fuel milage based on temp and speed. It is in the files --> 2004 prius simulations section. US is second tab in the spreadsheet. Pretty enough to print out. Not sure of the background of the calculations....

2004 Prius - Palm THS2 Simulation (SI, US & UK).xls
3D simulation (xyz, x=speed, y= temperature and z= fuel consumption results, 15x21 = 315 points, 15 temperatures and 21 speeds). By Didier LUSUARDI

 

You guys are killing me...

You want to talk mileage killer? Try driving in Michigan all fall/winter/spring and having to run the HEATER and DEFROSTERS!

These two (which I might add - unlike the electric AC) actually requires the ICE be running AND the AC compressor to run! Plus, the engine has to stay hot to keep the coolant warm!

It's a double whammy.

Proof here: www.w8kc.com/priusmileage.htm

 

temps make a huge difference. i figure i average close to ½ mpg more in the evening coming home (avg temp around 70-80º) then the mornings (avg temp 50-60º)

normal startup in morning... lose .2-.4 mpg on tank at beginning of drive. it slowly rises back after about 5-7 mins of driving.

in evening, lose .1-.2 mpg at first, then it rises back up, most times, im .1 to .2 mpg higher when i get home. my commute is about 8.7 miles. small changes in elevation, but overall change is only 35 feet.

i have only used air conditioning a few times this year and none were afterwork commutes. (i dont get off work until after 7-8 PM most days so i miss the hot part of the day)

and i have to agree with many here. i believe my mileage would continue to improve as long as i didnt use the air conditioning. now getting great mileage makes me feel great, but i dont think it will overcome driving in 90+º heat without air...

so for you, definitely look at congestion first before temps. your long trips getting 50+ mpg on the freeway is normal. getting any worse than that under any legal circumstances has to do with congestion, poor anticipation, or a desire to spend a lot of parking time at each red light.