See http://www.scangauge.com/, for a retrofit mpg guage for any car since 1996.
They sell one for $169, or $159 on Amazon. All I know about it is what I've seen in the ads. What it really needs is a big marketing push and a lot of buzz and word of mouth, appearances on Oprah, etc.
Imagine if everyone started getting immediate feedback on the effects of their driving! How much less oil would we need to import?
Maybe we can't all afford or justify buying a hybrid, but most people with cars could afford $159, if they can make it back in fuel savings in a year or so.