I've never managed to get close to 20 gallons in mine. I also have never got the gauge to register completely full no matter how much topping off I do.

 

It's been a while, but when we moved from N.C. to Florida, We did that 700 miles, pulling a UHaul trailer loaded top to bottom, and managed 21 miles per gallon on the trip computer. Pretty good if you ask me. As to the fuel tank -- early models all had the smaller tanks because the diesel models needed more room for the diesel additive tank. Later, they decided to fill the space for petrol vehicles by putting in a larger tank, using up the space taken by the blue additive for diesels. No diesel, more space for gasoline.