Hot Weather and MPG?

I've had my Gen III for a couple of months so far and have driven almost 4,000 miles. I have been practicing all the techniques on maximizing mpg and feel pretty comfortable w/ how to maximize mpg. In the city, I'm averaging around 51 mpg. Not bad at all!

My question is this: Does hot weather have an affect on overall mpg? I've read threads on this board where people talk about easily getting 55+ mpg in flat areas, but I just can't seem to get there, despite the fact the Phoenix is VERY flat. However, it has been over 100 degrees everyday since I've owned this car (usually around 110 degrees outside.)

Does the heat have an affect on mpg?

All things equal, the warmer the outside air temperature, the better the fuel economy. For most folks, however, all things will not be equal.

I assume you're running AC. AC affects fuel economy, especially at low speeds in the Prius, and the hotter the day the harder it has to work. I'd say in 100F+ heat in city driving, 51 MPG is respectable.

Also, as the hybrid battery gets above a certain temperature the car starts to curtail its use (heat is an enemy of battery life). I take several steps to avoid heating of the battery during the day: I have window tinting, and I park in the shade (probably most important), crack the windows, and use a windshield sunshade. It's a lot easier to prevent it from heating than to cool it while driving in a hot environment.

azgringo,

What temp do you usually set your A/C at? Do you use the "auto" setting?
I like to set mine on auto at around 74. If it's a particularly hot day outside,
I'll start it off colder, just to cool the interior quicker... but then I back off
and raise the temp back up several degrees.

I don't think that the A/C will play THAT big of a role in MPG... at least not nearly as much as your driving style. I'm starting to get the hang of it, and have definitely noticed my average MPG increase after a week of owning/driving the car.

I have a 2010 IV and live in Ohio (somewhat hilly), drive 35 miles one way to work and half of that on expressway. If I'm somewhat careful, like yesterday, i can get 64MPG. That's without AC. But with AC I'm still getting 60MPG. I really think if you get the on pedal off pedal technique down you can get this kind of mileage. You also get better mileage with the windows closed as it cuts down on drag.

That kind of dry heat effects all cars. No water vapor to conduct the heat away. At city/rush hour(s) speeds your radiator fan and electric coolant pump are likely running full speed with a/c and only a little less with out it. But 51 mpg is still 2.5x what my truck got when I lived in Flag. Phoenix looks much better and somewhat cooler(90f) at night.

When I first got on these forums, I saw a posting where someone mentioned that they set the temp to 78 degrees, and I thought, "What a crazy! Sweating like that for mileage!"

THen I tried it. At 78 degrees with recirculation on, it's quite comfortable even in 97-degree heat. I don't know how high the outside temperature can go, but that was not even with the vents aimed at me, but rather past me so I felt a light breeze.]]

78 in the car, having air flowing right past you is much more comfortable than 78 in a house.

I seem to remember that 78 was a sweet spot and setting the temp higher doesn't help much, but who knows.

I definately saw a reduction in mpg while in the Buckeye, AZ area for two weeks at temps around 108+/-. The drive to AZ thru the Great Basin of Nevada, etc. averaged around 51.5 mpg = highway driving. In and around Buckeye that dropped to under 49 mpg. whereas one would expect it to go up wouldn't one? My A/C was set between 72 & 74 degrees.