Eco-Boost Fuel savings will show up primarily in highway driving rather than city driving. When you have a small engine working hard at lower RPMs (when not in boost like in highway driving without a heavy load in the bed), the throttle is open further than on a bigger engine, and this reduces pumping losses. Pumping losses are significant for gasoline engines that are throttled, especially big V-8s that have the throttles near closed most of the time especially during relatively lower RPM highway driving when the engine pretty much loafs. The smaller turbo engines can deliver decent around town mileage too, but if you like getting into the turbo a lot for brisk acceleration just because you can, then the mileage suffers around town. Driving habits greatly affect mileage on smaller turbo engines in vehicles compared to non-turbo engines. The main advantage to diesel engines over gasoline engines is that they do not have throttles at all, and thus have no pumping losses. That combined with a little more energy in a gallon of diesel compared to gasoline is why diesels generally get significantly better mileage than gasoline engines. Witness the 2015 Ram 1500 diesel that is rated at 29 mpg highway, best in class. Downsized turbo engines could deliver even better mileage than today if our base gasoline octane were 91 and our premium octane was closer to 98, but the oil companies have fought this hard (maximize profits) and the EPA won't budge on this issue because of the backlash that would come from people complaining about fuel cost increases even though it should be a few pennies per gallon more unless the oil companies take advantage of the situation. And any small increase in the cost of base high octane gasoline would be offset by the better mileage on advanced high pressure downsized turbo engines. Europe is much more enlightened on this issue than here in the U.S. Also, Ford has amply demonstrated that downsized turbo engines today will go just as far as any gasoline engine (just as durable) - the technology is there to do it.
Steve
