Fuel tank

This will probably sound like a stupid question but here goes. I've got a 98 Silverado Z71 with ext. cab and short bed. Does anyone know what the fuel tank size is? I keep finding info that says it's 26 gallons but according to the fule gauge when it is on empty I only tank on 20 gallons. Does the gauge read short so as to not allow you to run out leaving a 6 gallon reserve?

Thanks
Frank

Yes