How 'bout:
"I've got a natural Hy" or "Drive Hy" - um, okay maybe not...
"YEE-HAH" - yes, this is the name I gave my car, but with a V6, I think it is appropriate (maybe only those of us in TX will get it though

)
or something a little more low brow - "Passing Gas in my new Hybrid" or "I pass gas - How 'bout you?" (sorry, it's late...)
or maybe you could market a part of an extension cord that will stick out of the gas tank

(you know, like the tiger tail) Of course you would have to make it so that it wouldn't scratch the paint
P.S. I would love to buy the lic. plate bracket - but they are illegal here in TX now since they "cover up" the name of the state up at the top - and I have this on good authority since it was a cop who had to tell me this (and regrettably, that was NOT the reason he pulled me over). These politicians need to find a better way to spend my tax dollars...
(and I am now hoping I have enough posts to qualify

)