Possible Clues:

• It's opened for a fill-up
• Top removed by attendants
• It's removed for a fill-up (2 words)
• Something to unscrew on an auto
• Item replaced after a fill-up
• Tank top often removed in public?
• It's opened at the pump
• Twisted part of an Impala?
• A loose one may activate the "Check engine" light
• Tank cover