Typical affect heater has on MPG?

Yes, as the previous poster noted, whether the engine turns on or off depend BOTH on the temp you select in the cabin as well as how warmed up the engine is. Even a low cabin temp of 70 degrees will cause the engine to turn on -when it otherwise wouldn't- if it is not properly warmed up; conversely a warmed up engine -after a long highway drive, for example- would easily sustain 80+ degree temps while remaining off.

Also:

(5) Another reason MPG drops in cold temps is the deflation of your tires, usually at the onset of cold weather. This can be avoided by filling them with NITROGEN, as opposed to regular air. Nitrogen is much less likely to leak out, and will do a better job of maintaining the proper Psi. Generally, you will have to go to a Tire shop to get a nitrogen fill as most of the coin-operatated machines at gas stations just fill with regular air.
You can get the Psi requirements for the Prius from the door jamb of the drivers side door. I generally like to fill each tire about 1-2psi more than recommended, when it is cold outside, to compensate for any deflation later in the winter. To get an accurate reading of what your Psi is, I'd recommend buying a handheld digital tire pressure gauge. To get the most accurate reading, measure your Psi when the tires are COLD, and the car has been sitting there a while .....in the morning before your commute, for example. (measuring them after car has been driven will tell you Psi is higher than it really is, since driving increases temp/pressure in tire)

 

Nitrogen fill is a scam - N2 diffuses at essentially the same rate as air, regardless of temperature.

The reason tire pressure goes down in winter is just the normal behavior of gases as described in the ideal gas law:

PV = nRT

Assuming everything else is held constant, if T goes down, so does pressure. To maintain constant P when T goes down, you need to compensate by increasing n (= add more air).

 

Grill blocking and a Scan Gauge are two of your best friends in the cold weather months. Mind you, it isn't so cold in my area (mid 30'sF in the morning leaving for work, arriving in the mid 40's F) AND I have a longish commute (30+ miles) of mostly freeway driving. But when my scan gauge shows I'm above 159F (typically I'll start the heater at 165F) I'm on the freeway and the heater goes on and stays on. My commute involves a long uphill climb then a much longer, gradual descent (where I lose about 400 feet elevation). On the descent, I either turn the heater off or greatly reduce the heat (from a setting of 80F to 70F) such that the Fan decreases to a single bar or two. If I know I'm hitting slow and go traffic, the heater goes off, because the slow and go traffic will reduce the water temperature. Once I'm moving again at a more consistent speed, the heater goes on.

The grill blocking helps the engine heat up more quickly and retain heat (by reducing the volume of cold air hitting the radiator). The scan gauge helps me know when I can turn on the heater and when I need to turn it off. This should be helping my mileage because the cabin is warmer and that warmer air is helping warm up the big battery.

 

Incorrect. Tires operate in the real world, not in laboratory test conditions. Your formula is pure theory. Factors such as Nitrogen being DRY while regular air attract more moisture, matter a lot. See:
Nitrogen vs Air In Tires - Why Nitrogen in Tires - Popular Mechanics

 

A: Sort of. From the top: Air is 78 percent nitrogen, just under 21 percent oxygen, and the rest is water vapor, CO2 and small concentrations of noble gases such as neon and argon. We can ignore the other gases.
Please note: - Nowhere does he say that regular air "attracts" more moisture. So, the author's chief concern is that the compressed air may contain more moisture than compressed Nitrogen. If we assume that compressed is is dry, the amount of water vapor in the tire is likely neglible.

Later on in the article, he mentions doing up someone's tires (not his) and finding quarts of liquid in them. He never stated what type of vehicle he found this on (passenger car vs industrial dirt hauler), and what conditions the vehicle was used/stored before he changed the tires.

Links for fun and knowledge
(How much air is in a car tire)
How many cubic feet of air is in an automobile tire? | Answerbag

(How to determine how much air is in an air compression system)
http://www.omega.com/auto/pdf/EngTips_airWaterSys.pdf

While the Ideal Gas Law is theoretical, it translates well in the real world. It shouldn't be dismissed so easily.

 

BUT... you can't adjust your pressure at home when it's filled with nitro! I prefer to "tinker" with the levels!

 

Yeah, that's one of the downsides
Every time air is low, you gotta make an appointment with they guys who have the Nitrogen machine.
Good thing, it usually only takes 10-15 minutes if their not busy....and usually gets me through the winter.

 

PV=nRT like another said. Schools try to teach you that equation starting in elementary school.

Nitrogen in tires comes in to play at hot temperatures, not at cold. It's something the NASCAR fans latched on to. Normal compressed air has moisture. When you are on a race track, steam tables come in to play when tires get over 100C. Once the moisture starts to boil, tire pressure escalates. I'm sure the mechanical engineers out there with a few thermodynamic classes understand what I'm saying.

Otherwise, nitrogen doesn't defy the ideal gas equation and the large molecule thing is something like less that 1 psi per year. If you aren't running around 200 mph in hundred degree heat, you don't need nitrogen in your tires.

 

This is my strategy for cold mornings; I'll start out with the heater set to OFF and my heated seats ON. By the time I drive out the gates and get to the first stop sign outside my community, I'll have driven just over 2 miles - about 5 min and if my ICE auto-stops, then I turn on the heater once under way again. For the rest of my 22 mile commute I have heat without affecting my mileage. I also grill block. Be careful though - if your daily temps start going over 60, remove the grill block to be safe.

 

My commute has a good but if hills in some parts. I sometimes try leaving the the heat low or off until I have to go up a long one. I know I'm using the ICE and trashing mpg, so then I turn the heat on about 75 and fan way up. In a minute or 2, I'm toasty again. I turn it off when going down them. It's hard to say but I think it may actually work, getting about 4 or 5 more mpg average on a 100 mile ride, mixed highway and backcountry roads.

I miss the heat dias I have on the Sienna and old Corollas, and all the older cars where I could turn off the heat completely but the wind would still blow enough air into the cabin at whatever temp the dial was at. This car, the hvac seems to be truly completely off, no cheating.