If you have a Honda Insight or Toyota Prius (and probably others), the car's display tells you how good you are doing for milage. We have found a 'sweet spot' at about 50-60 MPH without sudden changes in speed gets us about 50 MPG on our Honda Insight. However, going highway speeds (70+) reduces the MPG to about 45. Hills and headwinds can further reduce that.

I had an old '87 BMW which had an "instant" MPG gauge . . . it was initially very distracting . . . but ended up impacting my driving quite a bit. Hard acceleration off the line would net me 1 or 2 MPG . . . while coasting downhill would peg the needle in the 100 MPG range. It ended up being fun and informative. I appreciate the feedback the modern cars give you . . . the Honda Insight awards "green leaves" as "rewards" for efficient driving. They made a game out of it.