Recent MPG Issue
 

So over the course of the last month my part of Idaho has been hit with chilly weather and some snow fall. This prompted me to use my rear window defrost/mirror defroster as well as turn the slider to outside air and use my defrost/floor setting on the knob. I have just let them sit like this and never thought anything of it...this leads me to my problem.

I took a trip on the freeway last night and kept a close eye on my real time and trip a average mpg. The real time never once went more than a quarter of the way up, the average stayed around 27 and I never hit green on my in dash indicators. During that trip, on the way home my gas light came on and for the first time I filled up with premium. Today I had to make another freeway trip and kept a close eye on my readings and they still didn't get any better. At this point on the drive I for some reason turn off my window defrosters, slide the vent to circulate and turn the knob to vents. After about a mile of drive time my number shoot back up and at this point all the numbers and averages are back to where they should be.

Anyone else encounter this? I am going to see the dealership ASAP to have it diagnosed. :(

So I understand, you were driving with the thermostat set to recirculate with the defroster on? Why would you do that? That's how you make an incredibly humid environment, so the AC compressor was basically having to run non-stop to fight the humidity created by closing off your car from outside air.

The recirculate function should only be used when you need to very quickly heat or cool the cabin, after optimum temperature is achieved, you should immediately change it back to circulate.

Another note would be to check your tire pressure. As the temperature drops I know that I've lost several PSI.

Using the defroster will have a negative impact on mpg on all cars, as will cold weather in general. See here for more info: http://www.fueleconomy.gov/feg/coldweather.shtml

He had it set to outside air when he was using the defroster. Most cars won't even allow you to put the AC in defrost mode while set to recirculate because of what you said.

No, according to his post he had it set to recirculate, and then changed the settings by turning off the defroster and changing the airflow to circulate according to the OP.

"At this point on the drive I for some reason turn off my window defrosters, slide the vent to circulate and turn the knob to vents."

The fit will allow the air to be set to circulate with the defroster on to quickly heat the car on cold days.

Please reread his post.

I should have just done this in the first place.

This picture shows my settings when I was getting HORRIBLE mpg.
(First time using any of the defrost settings since owning the car.)

Attachment 13112

This picture shows my settings when the mpg went back to normal.

Attachment 13113

I also got a letter about a recall concerning the defroster that I will look into while I am at the dealership.

Rear Defroster takes 30 amps of power, which means drag on the alternator, and defroster via the vents uses the AC compressor also taking HP, simple logic means more load ont he engine, the more fuel is consumed.

Whats this recall with the defroster you speak of?

Heating and front defrosting when it is cold outside is almost free because it is a byproduct of engine operation and some (not all) free ram air from vehicle movement.

Cold air has much less moisture content than warm air, so when the cold air is taken in and warmed, it is warm / hot DRY air which is ideal for defrosting. This drier air also displaces the humid air already in the vehicle which is forced outside.

Rear window defrosting is a different story. The rear window is electrically heated and draws a lot of electrical power. This power is not free and is supplied by the alternator putting a load on the engine. So this additional load will affect gas mileage to some extent.

I do not know if the Fit does this, but most vehicles have a built timer that shuts off the rear defroster automatically when it is selected. I do not know the normal time out, but I'm guessing it is 10 or 15 minutes.

I would never use recirculate in the winter unless you find that the free heater supplied air is not capable of heating the cabin and also the interior of the cabin is completely dry. I have driven in -20s and -30s F. for extended periods in other vehicles and never needed recirculated air. I assume that the Fit heater is also capable of supplying an adequate amount of heated air at below 0 F. outside temperatures.

Air conditioning and summer defogging is a different story. Summertime defrosting requires the use of the AC compressor and evaporator. The AC compressor does not operate below temps of around 42 to 45 degrees intake air temperature but that is a different story for another posting.

Thanks for the input on climate controls folks, still doesn't really answer anything. The fit defroster had NOT turned off automatically, I noticed that.

Thanks JediPat for the posting. Who woulda thunk it would make such a difference in the mileage? Fortunately, here is sunny San Diego the defroster isn't much needed (my Fit lives in a garage) but I will be paying close attention when the A/C is turned on.

On my recent 17 hour trip from Central IL to Central GA, I experienced much the same kind of drop in mileage. Temps when I left were in the mid-upper 20s F with a light dusting of snow on the ground down to the St. Louis area, and mid-40s when I arrived after driving in light to moderate rain the rest of the trip up until the last hour or so.

I had been averaging around 38-40MPG, but the 3+ tanks I used on the trip averaged out to 35 MPG (my calculation). I was expecting much higher (42+) due to the extensive highway driving.

I had my HVAC and rear window defrost set exactly like your first picture, with exception that my temp knob was at the midpoint, not at full warm.

Would be interested to know more about the recall notice you got - can you post anything? There's nothing listed for this at owners.honda.com.

es

When you're running the defroster you're running the AC compressor. That puts added load on the motor since it has to turn the compressor and it's no longer free wheeling. Between that and the extra power draw from the alternator for the compressor, blower and the rear defroster means you will get worse gas mileage.

I avoid running AC or defrost when possible, but that's usually an option where I live... there, not so much :o

I don't think there's anything wrong... it's a small motor.

A 13-15% drop in fuel economy is not reasonable for just running the a/c compressor and a slight additional electrical load.

es

I am never surprised to see my mileage fall from 38-40 highway to 35 when running the AC in the summer FWIW.

Maybe I'm too used to driving the VW TDI diesels I owned for the past 14-15 years. Running the a/c never seemed to impact mileage at all. But then, I don't remember that big a hit with the '90 Honda Civic we had years ago, and our Chrysler-platform minivans never had that kind of hit for running a/c, either.

Just seems odd to have that big a drop.

es

I agree. The electrical load would only be about half a horsepower which might impact mileage by a percent or two.

The question I have is whether the mileage is truly being affected, or is the calibration of the on-board instrumentation being altered by the higher electrical loads? That instrumentation is there to amuse the owner, not provide laboratory accuracy.

Diesels like running under load, and minivans are big, sloppy, energy sucking machines. Either one is going to shrug off the additional load from the compressor.

My Dodge minivan's V-6 puts out 283 horsepower. Our little 1.5s with their 117 hp are going to notice the compressor a lot more, just like they notice every change in grade, wind and cargo.

Hey now, hey now, the 2015's make 130hp :rotfl:but yes, I agree.

My base '90 Honda Civic made all of 70 HP, and maybe saw a 1-2 MPG drop with the a/c on, if that (this car easily delivered low 40s on the highway, BTW). If indeed the 13-15% drop we're experiencing can be tied back to losses caused by running the a/c or defrost cycle, I still maintain that to be an abnormally high loss in fuel economy. YMMV, of course (so to speak).

es

Leaving your rear window/mirror defogger on forever will lead you to more problems than lower gas milage. It is frowned upon by the Honda Police and will get you in deep yogurt.
This was copied from page 129 of the 2015 Honda FIT owners manual ...

This system consumes a lot of power, so turn it off

when the window has been defogged.
Also, do not use the system for a long period when
the engine is idling. This may weaken the battery,

making it difficult to start the engine."

Ignorance of the Owner's Manual is no excuse: don't be Johnny Dum-Dum! :D