I don't think it's rude. It's not "up your(s)" in a nasty way. It's up/increase your mpg. That's how I read it, anyway.
Typical Prius smugness. It's like the bifurcated rearward view, Aveo seating appointments, and hear-no evil FM radio. You have to take the bad with the good. It's all part of the Prius experience.
Saw this license plate on a Prius driving to work recently: HAD A V8 New Prius (2011) owner as of 6 weeks ago and really love it.
I should put that on my new to me 03 Ford F150 CNG bi-fuel pickup. I drove it almost a thousand miles for equal or less money than the Prius and less pollution.
A friend USED to have plates that were 3M 3AT.....in your rear view mirror it says EAT ME....the BMV has banned it here in Indiana.