Found a Facebook ad some people might be intrigued by this. They offer a guarantee and 100% money back if you honestly don't like it. This was a Kickstarter campaign.
GOFAR analyses the data provided by your car's ECU and helps you make minute changes to the way you drive with simple real time feedback. If GOFAR is blue, you're good - when it starts to glow red, you're burning more fuel (and money) than is optimal.
It also pairs with your smartphone via an app to track everything from the fuel you burn to money spent and you can even workout when the best time to leave your home is.
This device connects to your OBD ( On-Board Diagnostic / OBD-II ) which is in most cars from 2006 onwards and will tell you what your engine light error means.
Wow thats one hell of a markup. You can get almost the same thing on eBay for $10. Your paying over $100 for that blue/red led.