thats really good to know! i am going to do it myself for now onI think they all used a 20 gallon tank beginning in the early '90s, regardless of engine type.
When I was in college and used to drive between my school and my girlfriend's I used to take my time filling up and get as much as I could in there. With my needle on E, and my OHC telling me I was a 0 miles until empty, I got 19.75 gallons in there. Yep, almost 20 gallons. I could get 19 gallons in there religiously when commuting between our schools, so it definately is possible.
The reserve comes on quite prematurely on these. Typically you'll have between 3-5 gallons when the light comes on, that's why most of the time when we fill up we average 14-16 gallons per fill-up. On that note, when you let the pump run fully open for a fast fillup, the tank becomes full of gas vapors/fumes and this will trigger the auto-off on the handles prematurely. These fumes will continue to trigger the auto-off when you squeeze it with your hands. When I was getting 19 gallons in regularly, I was pumping myself (PA), and would hold the handle at about a quarter of what "full speed" was, minimizing the fumes in the tank.