Usually around this time of year, when you use the heat, when you're at a standstill, your engine will warm up to heat the car and the MPG meter will go to 0 MPG

So when the gauge reads 0 MPG, what exactly is that supposed to mean?

Is the car just burning fuel indefinitely until it thinks it's warm enough? Is it burning more gas than when I would floor the gas (which usually shows MPG around 5-10)?

It means just that, 0 miles per gallon. Since you are not moving, if you sat there long enough to burn a gallon of gas you would have moved 0 miles. Therefore 0 miles /1 gallon = 0 mpg. When you floor it, you are moving so you always have some amount of mileage.

Right, I got that, but how much gas does your car use up when just heating up the engine for the heater?

It seems that every time I turn the heat off in the cold I get a better final MPG

I have a scan gauge on my TCH. If the engine is at operating temperature and sitting still at a red light. It's shows 0 mpg at .25 gallons per hour. I guess that don't seem like much gas but it sure pulls down the mpg.

I can't tell you the cold mpg while sitting still. I start my car and drive right off. No sitting still time on a cold engine if I can help it.

I read a long time ago, those that sit in the driveway to warm up the engine, are wasting gas. That really stuck with me. Although back then they were talking about carburated engines. Those big cubic inch engines were gas hogs when they were warming up. That may not be as bad with the improved fuel injection.

Our TCH engine is much the opposite of that. Still not a good idea to sit still to warm the engine. Their are times where the traction battery will need charging and the engine may run while sitting still.

It's a trade off...you might spend a buck or two (if that) extra every week or two on gas, but you will be warm and comfortable.
I say, don't look at the mileage meter, click the DISP button until you see the outside temperature. Then enjoy the nice warm feel of the car's heater.
Enjoy the car, stay warm and drive safely.
The cost is really not that big of a deal.

I use the block heater almost daily and plug it up on the back porch outlet about 3 hours before driving off. The car gets confused thinking it's warm and usually starts right up as soon as I press the power button. Although it is a little tiring to connect and disconnect it ever time the car is in the carport.

The good is that even when it's cold the temp gauge will read 1/8th to 1/4 scale depending on the outside temperature. I can drive about half mile and the engine is almost up to normal heat.

I found as soon as I start the engine I can turn the heat knob up to hi, just above 85 degrees. I leave it on the lowest fan setting, one bar. I do that till I have driven a few miles then may up it to the second bar till it feels warm enough in the car.
I back the temp down to 74/75 degrees once the cabin is warm enough.

The heater uses it's electric heat coils at first. When the engine is warm enough it switches over to heater core heat from the radiator coolant.

The electric heat coils should pull on the battery but I never really noticed it. I would think the coils would use the 12 volt deep cycle battery. The fan if turned up could pull some pretty strong amps.

Interesting how it starts with the coils and then switches to the engine.

For short trips, I usually just keep the seat warmer on, which heats up MUCH quicker.

Bingo!

Unless I'm on a LONG trip, I hardly use the heat at all.

1) I'm already wearing my winter coat. I don't want it 70 or I'll be sweating! (I wish you could set it below 65 for this very reason)

2) A toasty car has warm windows. Park the car. Let it snow. When you get back you now have frozen ice on the windows. Cool windows = no scraping! A quick brush and you are off...

I just went on a trip, non-stop, approximately 100 miles and averaging 65 mph. When I got to my desination I sat in my car for about 4 minutes with the car still running.

The MPG gauge stayed on 0 the whole time I was sitting in the parking lot.

I noticed while I am sitting still with the MPG gauge at 0, the AVG MPG per tank goes down, slowly.

Is it normal for the MPG gauge to stay on 0 after trips and to go down while sitting still?

If battery is running low, the engine will turn on to recharge it.
As you sit and the engine runs, the calculated MPG naturally goes down, since you are burning gas and going nowhere.