Hot Weather and 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.

 

I live in Palm Springs and since the average high temperatures in the summer (July) is about 108, I'm concerned about the impact on the hybrid battery life. Oftentimes it will get up to 111-117 and as high as 122 F. Since I recently relocating to the desert I've noticed that my in town driving mileage in the summer months has dropped from about 48 to 35 mpg. I can live with lower mileage for 3-4 months until it cools down but what is this doing to the hybrid battery life? Will it shorten the life of the battery or does it only impact the mpg performance?

 

I just spent 4 days in Palm Springs. I noticed a dip in mpg ( went from 46 down to 42) while in the desert heat. My Prius normally is in the Bay Area with cooler temps but going 70 on the freeways.

Heat will shorten the life of the battery. The best thing you can do would be to keep the car in the shade (ventilated garage for example) and avoid direct sun to lower the radiant heating effect. Since there are over 300 days of sun in Palm Springs, easier said than done, but excessive heat is the enemy of batteries. Your 12v battery may also see shorter life. How many miles on your Prius?

 

Another item to do would be to clean your HV fan. There is a thread about this with a nice video on how to do it. Having the HV fan running at its best will help with getting the cabin air recirculating.