Actually, I was thinking of an obnoxious rear display in 6 inch red LEDs, illuminating my instantaneous mpg whenever it was over 30 or so, just so those behind me knew. Kind of like the GH license plate holders, but instant and BIG.

"I am getting XX mpg RIGHT NOW!"

Illegal? maybe.
Annoying? yes.
Get me run off the road? very possibly.
Worth it to flash that while in front of a Suburban (or any American car)? absolutley.