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

And keep in mind if you have a tinted rear window, overheating the defroster can blister the film. I can't believe Honda didn't put a timer on this.

Great. Looks like I'm going to have to buy another mod. :D

es

Those look pretty cool! Can you just add the timer to our defogger switch? Am at work so I don't have time to read the FAQ's. I'm sure I'll be guilty of forgetting to shut my defogger off ... 1st car I ever had with rear window defogger!!

I would presume yes, but would have to dig into it to know for sure. The price isn't that bad…

es

I too forgot to turn the rear defogger off. I was like man how much did Honda save on this by not installing a 10-15 minute timer?? $2?

Soon enough I will have to start doing a pre-flight checklist before take off. lol

As I stated before in this discussion you need to separate the operation of the rear and front defrost / defog operations. The front and rear have separate controls and can be turned on and off separately.

The front defog / defrost operation will consume additional engine driven power over just normal heater operation IF the OAT / intake temperature is over 40-45 degrees. Below that temperature the compressor does NOT run. It is automatically shut down to prevent the evaporator from freezing up. If it were to freeze up you would have no air flow through both the heater and the defroster and you would have to wait until the next thaw with warmer air temperatures for the ice to melt out of the evaporator.

The rear defrost / defogger is a completely separate operation from the front. It is totally electric and consumes 25 to 40 amps. In most vehicles if you have rear defogger on you also automatically have the heated mirrors operating for some additional electrical load.

In the picture posted by the OP, the rear defogger is on as indicated by the lighted indicator in the "rear" button.

So in the picture you have one and possibly two additional power drags on the engine.
1. You definitely have the additional electrical drag of the rear defroster and the higher voltage triggered by the ELD (Electrical Load Detector).
2. You also may have the additional drag of the AC compressor depending on the intake air temperature.

As a side note, if the fresh/recirculate lever were in recirculate (it is not in the picture) then the intake air would be coming from the cabin and would be above 45 degrees F. once the cabin (inside) air temperature warmed above 45 F. If it was in recirculate mode we would we would know for sure that the compressor was running.

One trick that usually works for me is to move the right airflow dial to the open space between the Floor and Defrost/Floor setting. You can hear the the A/C compressor come on as you approach the Defrost/floor setting, so just back the dial off a bit til the compressor goes off--your dial will be about 2/3's of the way to Defrost/Floor, but not all the way there. Air will still be directed out of the Defrost/floor vents, but you do not get the benefit of the A/C dehumidifying the air. it's a reasonable compromise when you don't have heavy fogging.

Except for the fan speed being way too low, your first picture is how you should have the settings for clearing fogged windows. But that's temporary.

Once they're cleared, you should cut the heat back to what's comfortable--about halfway, redirect the air to the dash and floor vents, and turn off the rear defroster. You might be able cut the fan speed back from 3 to 2, but leave the air source set to outside air. You need to have air exchange to get the moisture you generate out of the cabin.

It sounds like everyone is trying to scoot around the issue at hand here. I thank you all for your advice, I am taking it to Honda today. After a brief description of the issue they wanted to see it as soon as possible...this is not an issue with how my controls are set, its an issue with excessive fuel consumption under a certain setting...way more than I could mess up with running my AC or defrost too much.

I will update you all on the issue after I get back. Thanks again for the input.

If my settings are what is going to stick with you all for some reason then please keep in mind this summer I kept the AC full blast and was still getting 42-48 mpg, I know my car well enough to not be a "johnny dum dum..." or whatever else. This isn't an operator error.

I don't think any of us thought you were being a "johnny dum dum" or anything. We just wanted to make sure you were aware that you will always get lower MPG in the winter due to various reasons (colder air, lower tire pressure, using the rear defroster, heated mirrors, heated seats, etc). You never stated what your MPG was before you noticed the drop, just that it was lower. Now that I see you are talking about a very significant drop from 48 to 27, I agree something is wrong and this isn't a typical drop in MPG due to it being winter.

^This.

Yeah, that much of a drop is a problem.

Foot in Mouth ... AGAIN!!!
 

Johnny Dum Dum not directed at the OP (JediPat) but directed at anyone who fails to read the owners manual. It certainly wasn't my intention to insult your intelligence. Wanted to give a "heads up" of warning in owner's manual about leaving rear window defogger on for extended periods of time.

My attempts at internet humor always backfire. I sincerely apologize.

Good luck with the dealership and I look forward to reading about your resolution

I appreciate what you wrote, as I haven't fully read my owner's manual and might just be called Johnny Dum Dum :-)

This is the first car I've ever owned, and I grew up in a very different climate... I don't even know if we had a button in the car to warm the back window? If we did we never used it.

I had no idea it shouldn't always be on... my usual routine is to switch on the front and rear defroster and leave it on there!!!!!!! So thank you & everyone else posting here for educating me! It also didn't occur to me that keeping it on re-circ would up the humidity (duh! why didn't I think of that??) and mean I would need to leave the front defrost on...

I, too, have had a drop in gas mileage, but it's no a huge bump, and due to things like me not driving as carefully for best fuel economy as when the car was brand new, "winter mix" (just learned about that from here... no one in person I know has heard about "winter mix" fuel), haven't checked tire pressure & have driven it almost 2,800 miles in about 6 weeks, weather has changed from 50s to 0F to 30F range. I don't even know if it matters which gas station I use to fuel up... I feel like gas quality should all be about the same but I have no idea. Also our state (Minnesota) requires ethanol to be in all gas :-/

Anyhow, I just wanted to say thank you to everyone who has posted here for educating me, and also to the OP--I hope you find out what is causing such a drastic change in fuel economy! And I hope it gets fixed quickly!

Happy Thanksgiving to all!

I took it in and they brushed it off as a cold weather thing. They check my fuel trim and everything was running well at the time. They wanted my car for a week to test but I have a hard time with that right now. My buddy from Honda r&d said I was 1 of 2 people in the country to have this issue...he is waiting on info from Canada as well.

Kind of bummed...but oh well.

Bummer! Well hopefully Honda can figure out what is causing this issue (sounds like they don't know since they wanted the car for a week) and get this fixed for you... or maybe they just trade out your car (lemon??) for a brand new one...? ;-D

Keep us posted & good luck!

I found out with my 2013 that in either summer or winter I had to keep my front defroster on all the time, so never noticed a drop in mileage as I never drove without it on. I have the 2015 front defroster on constantly too. Was curious how that would work as I have the dx trim model and it has no A/C. Seems to work fine so far.


Canada has had a quality control issue with NO rear defroster and no heater for side mirrors in the dx trim model - was just informed that the parts were in and will get it fixed this Friday. So, can't comment on that for lowering mileage, or km in Canada:D! Will see once they've installed the missing parts....wonder what else they've missed..:popc:

Most cars do it but I don't think the 3d gen fit shuts off the rear defrogger automatically. Odd that it turns the AC compressor on automatically when you turn the vent knob to defrost (though unfortunately it doesn't turn the green AC indicator LED on to inform you about what it's doing) most cars don't do that - yet it frogettes to turn the rear defrogger off automatically after its been on for 15 minutes. Oh well.

Had no idea the AC turns on with the defrost- and I was wondering why my mileage had been low lately.

I would think you could add a timer anywhere in the circuit. If it has a fixed, non-adjustable time, it could be very small and put in an out-of-reach spot.

It's pretty common, at least on the cars I've owned. The AC pulls the moisture out of the air.

If you search, there is a thread where you can pull one wire and disable this "feature". You'll have to manually push in the AC button if you want it on when running defrost.