It seems to me only fair to reward faithful readers of R World with the occasional million dollar idea so, here goes.
Sell "hybrid" metal decals for people to stick onto their cars. Even though this wouldn't change their actual mpg or carbon footprint, it nonetheless would do a great deal to impress the neighbors. Personally, I look forward to being able to buy one to stick onto my 1964 Rambler. I know that the guy down the block would like one for his Hummer.