All it would take is one aware (a.k.a. hybrid) owner to put 15 gallons into their 13.2 gallon tank and it's over. Could you imagine the lawsuit?
Gas companies rip us off plenty in front of our face. I doubt they'd resort to such tactics when they can just raise the price when they want for whatever reason they want. All they need is some penguin catastrophe in Antarctica and that will somehow disrupt the flow of oil and raise prices .25 a gallon...

Oops there's cold spell in Canada - there goes another .10...