drastic MPG drop with heater/defroster on

yup but your overall tank average will only drop 10% or so. you can't go by temporary mfd readings. thank the stars you're in florida!

 

Settings you should use when driving in cold temperature before engine has warmed.

Slowest fan speed is enough to keep windows from fogging with one or two peoples in car and less it plows less it’s going to steal heat from the engine.
Smallest temperature number less heat it’s going to steal from the engine.
Use window+feet because it will blow enough to windows to keep them from fogging but it doesn’t require engine to be as warm as front window heating mode.
AC off cause when AC is on part of air is only going through AC and not heater so heater has to heat the heated part of air hotter than other vice. This will require engine to be warmer.
If system requires engine to be warmer it runs constantly and makes fuel economy worse.

With these setting I can get the same fuel economy of 4.2L/100km or 56MPG (actual) in 16km or 10Mile trip in 32c or 90F and in -1c or 30F

 

How long is your commute? The shorter the commute, the bigger impact the cold will have on your mileage.

Also, most people run lower pressure in the rear tires, any reason why you're doing it in the front?

 

Are you parked outside, having to run the car and defogger for some time? That can be a real killer on short trips.

 

I don’t think Florida ever gets so cold that you have to run it for longer that 10s or so.

 

It's because your heater needs so much energy the engine will not turn off at the normal spots on your commute. I.e. Sitting at a stoplight or coasting down a hill. It's always turning to provide energy to the heating system. It's like disabling the start stop system in modern cars. You're going to take a fuel efficiency hit.

 

The tyre pressure are also wrong for economy and tyre ware. They should be at 40 psi front 38 psi rear or even couple of pounds higher all round.

John (Britprius)

 

good catch john, who keeps their tires at 35f and 38r? your bass ackwards.

 

I just keep them all at the same psi, simplifies thing when rotating them.

 

What kind of mpg did you get last year around this time of year?

 

60-65.

 

It's likely due to the heat/defrost settings. However, even being really BAD with those I've never managed below 35mpg. There may still be something wrong. Do what's been suggested here and get back to us.

Also, I've noticed if I go relatively straight to the freeway, the mpg hit is a lot lower than if I'm driving city in the cold. Heat also warms up a lot faster. Of course it's a couple minutes to the freeway so I'm not jumping down a ramp (hitting 3k+ rpm) with a completely cold engine. If that option's available (some places I go don't have a highway route to them), I'll try to leave the heat off until I'm up to speed on the highway so the engine warms faster.

 

Keep the heat set to "auto" and the temp anywhere from 69-71, run the a/c only when needed by toggling the button. That's the sweet spot for mpg's. Otherwise, put it hotter and the ICE will run much more and harder to create heat.

 

You sure there's nothing wrong with your ride??? 44 to 27 is a huge drop. I can get 28-29 mpg highway with the current gen Avalon V6.

There are 2 ways of the defrosting the front windshield - heated dry air or cooled dry air. In Florida where humidity is high most of the time, I find it much faster using the AC to defrost. As for the rear, turn on the wiper and crack the rear windows open to neutralize humidity.

 

ive noticed a drop of about 8 mpg now that the cold/wet weather has set in, sometimes that gets as bad as 10mpg.

Im running mine all day, as a taxi you pick up all sorts and the worse ones are those who are wet, the car fogs up in seconds so i have the blower on 2 and the heated rear window running, both of these draw lots of current.

Roll on the spring when its back to 58/60mpg

 

In the UK, the biggest contributor to MPG loss in winter is the change to winter formulation petrol. Using Fuelly (or similar recording method) to record your fuel use, you can pinpoint when this occurs. You can also see the converse, when the petrol formulation is changed back to the summer formula.

Once outside air temperatures fall consistently below ~8℃ (~46℉) then you will see the effects of temperature and thicker air contributing lower MPG's.