SRJ - 30 years of history have solved this for GM (the cylinder deactivation thing).
That feature is widely available on their V8 trucks and SUV's. My Brother in Law has it on his suburban (7 children) and he says that you dont notice it unless you are watching the display.
Im sure that it helps fuel economy (V8's of today are getting into the mid-20s on the highway) but you could argue that its usefulness is limited.

-John

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2009 Kia Sedona
2009 Honda Odyssey EX-L
12006 Jetta Pumpe Duse
(insert Mercedes here)

Husband, Father, sometimes friend =)