Re: (blackTT55)
I totally believe the 30+ mpg numbers. The day my mother-in-law died, I tanked up at the Burbank Costco and drove to Palm Springs with cruise control set at 68, A/C off. Only had to drop out of cruse-control three times. (Had OEM Michelin MXM-4 tires set at max recommended inflation of 38 psi. ) Trip average was 33.5 mpg.
Remember that the ECU is running the boost near maximum psi during highway cruise but they back off on fuel flow and spark timing to prevent pre-ignition, That way the boost is up when you romp on it so all the ECU has to do is advance fuel flow and spark timing and you get near-instantaneous response with little turbo-lag. the advantage is that the high boost keeps torque high (i.e., lowers rpm) at cruise at 65 mph so the efficiency really is nearly that of a 2.3l 4-banger.
One thing I noticed, though was that tires can make a difference. my wife's S80 dropped a couple mpg when we went to the Eagle F1-GS-D3's Great tire, very sticky, but performance comes at a price in mpg. I just put on some Michelin Exalto A/S on the S60 (215-55-16) and mileage is about 1/2 mpg or so worse than the MXM4's.
But if you have a lead foot, your mileage goes out the window. If you drive the car right, you'll get good mpg. It's your choice. Try it one night--take a road trip to nowhere on a flat stretch at 1 AM when the traffic is light. Set the cruise control, leave the A/C off, and see what you get.