Window Fog and MPG

Some days, with heavy rain, there's no way out: AC is the only way to stay safe. Other days you can dodge it.

If you've been running AC and then switch it off, you WILL get a humidity dump. If you manage to get through it it'll involve having both front windows cracked, mode in heat/defog and moderate to higher fan speed.

It can take up to 10 minutes to stabilize. It's somewhat akin to withdrawal from an addictive drug. Again, don't push TOO hard: if it's heavy rain and badly fogging, might be best to leave it to another day.

One thing, try to shut AC off about a mile before your destination, give the humidity a chance to dissipate.

 

Being able to see is more important than getting absolute maximum MPG. Just leave some form of ventilation turned on full time.

If windows are fogging up, favor outside air instead of recirculation. This may (often, but not always) allow you to keep the windows from fogging without having the AC compressor run.

It may also be possible that you have a bunch of moisture unintentionally stored inside the vehicle, increasing fogging. Remove all such moisture stores, take floormats inside to the house to dry them out, do the same with any towels you use to wipe fog off the windows. Take out wet boots or coats. If any carpets are wet, take the car inside the garage to dry out as best as possible, opening windows and using a fan as necessary. Look for any water under the hatch floor, wipe and dry it out as necessary.

You may want to use some sort of desiccant in the car. My spouse uses a bag of DampRid in her ancient leaky car. When she recently filled a fresh bag much too quickly, I found that tree debris had clogged the rain gutter under and around the rear hatch (invisible when hatch closed), causing water to overflow the gutter directly into the vehicle, collecting mostly under the spare tire. Dry out as above. Look for any other leaks.

In the end, saving fuel by turning of de-fogging ventilation is little different than by leaving headlights turned off at night, as is done in some parts of the world. Comparatively little fuel is saved by turning off the device. The vast majority of fuel savings comes from wrecking the vehicle and killing the occupants, thereby ending the need for fuel at all.

 

I set mine for outside air to come in (not recirculate) and set temp to all the way to HI (not fan speed) before I drive off. Once I drive off, the wind outside will enter the cabin at warm or hotter temp so there's no need for fan or heater to be on.

It doesn't get colder than 33 degrees here so I can get away not turn heater in AUTO to keep windows clear.

 

I can not believe that turning the heater on would lead to 5% more consumption. Where do you get that idea from?
The AC does use power, but when it's not very hot outside, that's also just marginal.

Using the interior fan surely does not lead to anywhere near 5% extra consumption.

 

You left out: 1.5 Active ventilation from outside without added heat. I never set my climate control lower than this.

I also don't know where this 5% per step claim comes from. Both heat and AC should have highly variable fuel consumptions, depending on conditions and settings, and I tend to run both for as weak of conditioning as I can tolerate. Much of the time, I can get away with no added heat or cooling, just fresh outside air.

 

No it doesn't. You are WAY overestimating the impact of the HVAC system on your mileage.

In most situations, all you need to keep "fog" from the inside of the windows is a little air movement from the outside.
The easiest way to do that is: Keep the HVAC system on AUTO and the RECIRC off.

The impact on your mileage will be minimal except when outside conditions are extreme........and then you can choose to suffer or let the system do it's job.

What you should NOT be doing is screwing around with the controls constantly and taking your attention away from DRIVING SAFELY.

 

You can get that 5% back by removing the rear wipe blade.

 

I've noticed when partially warmed up, turning the vent system off when stopped at red light, can trigger the engine shutdown. Once the engine's more fully warmed it'll shut down either way.

 

Just a couple test runs isn't enough to separate out all the many variables.

The ICE auto-shutoff depends on engine coolant temperature, and having any climate control turned on raises that temperature threshold. So during engine warmup when there are opportunities to ICE auto-shutoff (e.g. waiting at stoplights, pulling up to stopsigns), the increased fuel consumption (from missing those opportunities) will be exaggerated compared to operations after full engine warmup.

I just leave the temperature set to LO. No added heat, no cabin cooling when AC is turned off (I don't use AUTO mode), and only minimal automatic use of the AC system for windshield defrosting. Some other readers here use different methods.

Do note that your existing on/off pattern may have left some moisture pooled inside the defroster system, which may take some time to flush out. E.g. in older generation cars, my mother was deathly afraid of turning the defroster on while driving because it would cause an instant fog-up from that pooled moisture. It then took a while to clear. So if she didn't remember to turn it on while parked and warming up (and she always had everything turned off unless expressly needed), she wouldn't use it later either for fear of the extra fog burst.

But in my cars I used the Defrost position, without heat and with fan set to slowest speed, as the default setting. So I didn't have the pooled moisture to cause that brief but intense burst of fog.

 

rain-x