I drove until being out of gas and I loaded 2 gallons dident run again

Get someone to draw out the contents of the fuel tank then refill with gas. Should be fine. Do not run the car anymore until you do. It could very well destroy the engine.

It varies for every car. Usually 1-1.5 gallons when the low fuel light comes on. I drove my last '93 Ford Taurus around until the gauge said it was empty. Lifted the car and took the gas tank off and it had 3 or more gallons still in it. Here's some fun, zero your odometer, fill the gas tank and drive it until the low fuel light comes on. Then drive it till it dies out, obviously somewhere near home or where someone can drop off a full gas can.

Check crankshaft sensor.

Possible emissions problem. get the ecm scanned & go from there.

You may have cooked the fuel pump.
Cars today use a fuel pump in the gas tank. The pump uses gasoline for both lube and to help keep the pump cool. Anytime the tank is below 1/4 the pump is working harder and running hotter than normal. If you drove it for 2 hours low on fuel, the pump may have died.
I would check the fuel pump relay to make sure the pump is getting power, if it is not, you can bypass the relay to verify it is not a pump problem.

1. Fill gas tank 2. Record odometer reading 3. Drive car as usual until 1/4 full. 4. Fill gas tank and record how many gallons it took to fill it again. 5. Record new odometer reading and subtract original reading. 6. Divide miles driven by gallons used.

First question is yes bad or worn plugs can burn more gas to generate the same amount of power.
Second question, you could fill up the tank and record the mileage, then fill it up again when it measures half a tank and compare.
If the gas gauge is working correctly it should take half the total number of gallons the tank holds to fill it up again. And the miles you drove divided by the number of gallons used equals your gas mileage.

low oil on some models will stop it from starting however I would say you have a fuel filter stoped up this happens sometimes when you run a car out of gas they pick up trash,

its vapor locking losen the fuel cap, and if that takes care of the problem relpace it!!1

Your fuel pump is sticking and not pumping, this is common if you run the fuel down to less than 1/4 full, the fuel pump needs the gas to cool itself, below a 1/4 tank the pump is not getting cooled and after repeatably running low on fuel it overheats and fails. Running the car near 1/2 tank extends the pump life to many years more than running tank below a 1/4 full..