McG -

There's a certain advantage and convenience in having the tank generally "near full" rather than generally "near empty."

The rationale that it "costs more" to keep it full just means that you drive it till it's empty before you add any. You don't really have to do that.

If you want "regularized expense" when you've used 20 put in 20, whether it's gallons, litres, $, €, £, ¥, or glass beads and lizard skins.

It makes no real difference if it's the top 20 in the tank or the bottom 20 in the tank.

Although the benefit is probably less than touted by some, a smaller air volume in the tank probably does reduce condensation and oxidation of fuel in the tank. An offsetting factor is that a full tank might be more attractive to thieves if they knew ti was full, but they really can't tell how much is in the tank when they punch the hole in it.

My real reason for prefering a "near full" average is that in a couple of very rare - but repeated - instances when the local population all went mad it was impossible to get enough fuel to get out of town.

I find it "inconvenient" to be forced to stop at the first fuel pit when there's a mob following, and besides the price may be better (and frequently is) a little further from my immediate neighborhood.

Having failed to follow my usual practice of keeping my fuel in the "top half," I did pay $78 (~£40?) at my last topping up, but it had been 97 days since I'd added any and I only had about 100 miles worth left in the tank. (I do keep an accurate log.)

John