Two tanks both with 19 gallons! Thats a potential of over 3000 miles per fuel up with todays efficientcy!
Dream on. Even the best of today's full sized pickups seldom get over 20 mpg on the highway at reasonable speeds. Compacts may get mid twenties. If you look through the pickups reported on Fuelly I expect you will find most averaging about 16 mpg (US gallons).
The listed volume could be the tank's total volume. These things aren't basic shapes though, and it is possible there are air pockets formed when it is filled. Those going for range records might use a jack to tilt the fill port up and allow more air to escape.
It is usual for fuel tanks to have an "unfillable" volume on the order of 10% to 15% of the total. The filler neck is so designed as to make the tank have part of its volume unfilled. This is to allow for fuel expansion without overflow. Depending on the location of the tank vent it may be possible to fill the rest of the volume, but no faster than the trapped air can vent. Diesel fuel has a nasty habit of foaming when the tank is rapidly filled, and this often shuts off the fuel nozzle well before the tank is full.
There is question whether the tank volume as listed in the manual is total volume, or "fillable" volume. It would not surprise me to learn different makers do it in different ways.
Airplanes often have a total fuel capacity listed in the manual, along with "usable fuel." Usable fuel may be less, and is affected by tank shapes, fuel pickup locations, and the fact that fuel may slosh around in the tank(s) during maneuvering, and thus move somewhere where the fuel pickup isn't. I have a boat which has the motorcycle system of a reserve tank. When the fuel gets near the reserve level, and the boat is in a left turn, fuel will flow away from the pickup and the engine will miss. Ah, the vagaries of fuel systems.
I NEVER EVER stop at the first click. If so I would only get half the tank full at times. The stop click depends so much on your tank filler line back pressure, and how bad the gas station hose filter is. I can sometimes get 10 more gallons of fuel into my gas guzzling full size Bronco when I ignore the first click. If I stopped at the first click, I would probably get a few 30+mpg tanks of fuel, which would be a roughly 50% error in actual fuel mileage. Sure, it would average out over time, but it wouldn't be right logging a 30 mpg tank when it was actually a 20 mpg tank.
I always keep going til it definitely won't go in anymore and I know it is deluxe full. Fill it til you see it.
In my days of destroying evaporative emissions equipment I had no trouble getting two extra gallons in most vehicles.
That's a big charcoal canister! In newer vehicles I think they have added some check valves or something. In my wife's crv when it's full it's full, can't get anymore in unless you fill to the cap. In the cressida I could spend all day filling up the evap system.
My normal system it till it cuts, then with it still all the way in one more hold till click, just to make sure the pump wasn't giving a false click the first time. I should stop that, it has only happened to me once or twice that it cut out a few gal early, and I was probably spacing out at the time and it was user error.
In my fiesta I have learned to stop at the first click. I pump on the slowest speed and when it clicks off the first time, I'm done. I tried going further and it only spilled on the ground, every time.
People with gasoline engines should never fill to the top of the neck. Its hazardous to do so, and a good portion of that gasoline will evaporate anyway. As for my diesel, I've vented it to allow for an extra two gallons (~16.5g), and consistently get 750-800 miles out of any given tank.