From http://www.mcclatchydc.com/cartoons/gallery/80178-a80399-t3.html

For example, I think all cars should be forced to:

• Show the user current MPG at ALL times, something that cannot be hidden or deactivated. It must also cope with more than 99.99 MPG – an extremely irritating limitation of both the Toyotas I have owned.
• Show the user current average MPG at ALL times.
• Add a red/yellow/green indicator lamp next to the current MPG and average MPG, so the driver is constantly aware of how they are driving. People who don’t study their MPG are often unaware of what is a good or bad figure for their particular vehicle.
• For manual transmissions, add a simple indicator suggesting a more appropriate gear if the efficiency is currently poor – eg “Try changing down to 4th”.

It can be very hard to drive economically even if you watch a standard MPG counter as so much depends on the temperature of the engine and incline of the road. Some reasonably simple logic and sensors could make the car educate people.

Something like this could easily become as ubiquitous as peoples’ reliance on navigation tools like Tom-Tom systems. A retro-fit kit would probably pay for itself within a year in fuel savings alone – so making them mandatory could be a real boon. Of course whether a retrofit is possible on older cars is another matter.