[MetroMPG]

Minimal. Definitely won't be detectable at the gas pump.

It could even make things WORSE if the plate location isn't smack in the middle of the stagnation point. In that case you'd have flow coming at that thing from above or below, not head on, making a bit of a mess.

But you're right: it'll get people thinking! Thinking... "if he hits a pedestrian with that thing, will it shatter or chop his leg off at the shin?" :P