Remember, US gallons are smaller than ours. That 25 mpg is only around 20 mpg in the US. As regards shutting down cylinders to improve milage, Cadillac did that back in the late 70's or early 80's with their 8/6/4 system in their early fuel injected cars, from memory, it made less than 2 mpg difference than just leaving it on 8 cylinders all the time.