USA Today ran a small story last week on Nissan Motors plans to equip all of its cars and trucks with a dashboard gauge showing the fuel-efficiency of one's driving. The gauge displays your instantaneously computed miles per gallon as a bar graph - the more fuel efficient you drive, the longer the bar displayed.
Nissan claims that based on its in-house testing drivers will cut their fuel by 10%.
I bet if the price per gallon of gasoline was also displayed, or maybe the IRS standard cost per mile reimbursement rate (currently 48.5 cents per mile) used instead, people would drive even less. Seeing that the drive to the local store ten miles away cost you $9.70 might give you incentive to do it less.
Maybe Nissan will add in a costing feature as well in the future. The average cost per gallon gasoline or a total cost of driving per mile could be broadcast over a preset radio frequency, which then could be used to compute the cost per trip.
Given that Nissan's gauge looks software driven, this shouldn't be too difficult too add.