I'd like to see how much longer my trip will take if I slow down by a few miles per hour. Most cars will proportionally consume more fuel the faster they go. Knowing how my ETA will be affected will help slow down a few mph which has many benefits beyond fuel savings. (The request to add a "how will my ETA be affected if I speed up" is a different request and probably will be harder to get past the lawyers... I'd be happy with a little bar graph on the side of the map that shows minutes added with reduced MPH...)