Missing 3-4 gallons.

You'll never get every drop or even every gallon out of one of these tanks. Just the nature of the pickup and tank will keep you from getting it all. You should be able to do better than 3 gallons, but I would say 1- 1.5 gallons is just par for the course. And yeah, the rake on the car could make a pretty significant difference, especially if the pick up isn't perfect.

You might be able to get closer if you're sitting in your driveway with the car level and the pickup sitting perfectly on the floor of the tank, but when you're driving everything is moving and sloshing and a little bit of air will sputter the engine.

The reproduction tanks are 16 gallons. Maybe there really were 18 gallon factory tanks, but, pretty much all the ones I've seen were 16.

I wouldn't drop the tank unless I knew it was full of junk. Check the sender to make sure the pickup is as low as it can get, and while the sender is out grab a mirror or something and have a look in the tank. If something in the tank is causing you to be several gallons shy of 16 it should be pretty obvious.