I recall my mom having several Cadillacs back 10-20 years ago that had a display that would tell mpg. It would display miles to empty, miles per gallon since last fill and miles per gallon since you set a particular point on a trip. In addition it would give instant mpg. This allowed the driver to bump up or down their speed to get the best fuel economy. Most of the time it just took adjusting the cruise control 1 or 2 mph up or down to get the best performance.