Minecarts got nothing to do with time, it counts tracks.
If you make a track like this () so it just goes round and round really fast the coal finish ups real quick. But if you
do a bigger circle it will last longer. because it doesn't go over as many tracks as it does in the small one.
Same with a straight line and a downhill one.
I'm not sure about if you can stack coal in it but i usually do no idea if it works
How would a cart run out of fuel any faster on a small circle than a large one? If it counts the number of track blocks it goes on, it should travel the same distance in the same amount of time, regardless of the shape of the track.
I think he mistook revolutions for distance. Silly person.
And no, you can't stack coal in minecarts. I recommend a booster; they don't use any coal. Use the search bar to find a good'n.
Actually, you can do multiple right-clicks on the Powered Minecart to keep it stocked with fuel for a while. I don't know whether you can have more than 64 coal in there at any one time, though. But I've just stocked my minecart once, and it's run quite a long distance (Up and down from the surface to the bottom of the map, about 15 times)
That's more than 1 coal could keep it powered for.