The best way to get good gas mileage out of any car is to keep it in tune, run clean fuel with no water(if you can find it), dont rabbit-start coming off from stops and always use overdrive on the freeway. As for the 8-6-4 caddilac systems, they sucked. The only good way to make those motors work is to convert them over to a traditional valvetrain. But as for the modern cylinder deactivation, doesnt it seem that your wasting a lot of energy moving the pistons on the deactivated cylinders. What(hypotheticly speaking) if you went with a W style engine a-la volkswagon. then, instead of shutting down just the valves, you actually freed up the cranks using a design similar to a full floating axle assembly or a hydraulic design. Then, instead of 4 active cylinders and 8 moving pistons you would have 4 active cylinders and 4 cylinders on standby. Is it Doable?