Actual Capacity of K/5 Belly Tank?

['68OrangeSunshine]

Anyone know the actual capacity of the stock gas tank in a '71 GMC Jimmy?
Owners' Manual states a nominal 20.5 US Gallons.
Last Weds evening, I ran out of gas.
[My fuel gauge doesn't function. I know the gauge itself is good, so it must be in the sender. The belly tank is protected by a huge skid plate held on by about 48 capscrews, with the authority of 52 years of rust-in-place. I usually use a log. But it's not infallable].
Roadside Rescue dispatched a driver, who told me he needed a dollar amount, for the gas, so I said get me $20 worth. At $3.599 that should have bought 5.55 gallons. He showed up with a plastic 5 gallon gas jug and funneled it into my tank. I gave him a Twenty. Truck started and I got to my meeting, 45 minutes late.
After the meeting, [about 3 - 5 miles later] I stopped at a station and pumped gas until it shut off from filling. I did not overfill or spill. Gas Pump registered 19.5 gallons.
So the math is off. I know I did not burn 4.5 gallons in 5 miles. 5+ 19.5= 24.5, which is beyond the capacity of the Blazer tank. What happenned to 4 gallons?