Fast drop in MPG's

[94Q45Beast]

What are the possible reasons that i'm getting almost 3 mpg. I noticed fuel below the gas cap "door" where it looked like it dripped onto or in front of the tire because I smelled it and felt it (I didn't physically see it drip), so common sense tells me that its a bad gas cap, but no CEL light, and I re tightened it
also the gas cap was checked almost a month ago during an emissions test.
also can the Michelin x radials handle the top speed of the q45?

1994 Q45 Base