Was also thinking about this over the weekend. Can't answer your question unfortunately, but am interested in the responses!
Around town, I always use manual as i can squeeze an extra 1-2mpg fuel economy compared to D, but i guess this is more related to the fact that i know when the road is straight and clear so can shift up earlier and avoid prolonged 3k+ revs when there's no need, especially downhill. I don't hang about and certainly don't change up before 2k revs to avoid labouring the engine
When cruising on the motorway (usually on cruise control), I tend to stick it into S and then manually knock it into 6th - if left in "D" it has a tendency to drop back into 5th whenever i make minor speed adjustments using the steering wheel, or hit a long incline. Given i'm usually well over 2.25k revs when in 6th, I can't see the need for the box to drop down and it is quite annoying to get the kick in the back for only a few seconds before it shifts up again.
On the 1200 miles (80% Mway, 20% A/B road) i did this weekend, i managed 22.2mpg from an 07 Jag 4.4 petrol lump (calculated from tank fills, not from the OBC) at an average speed reading of 58.5mph. I'm happy with that!
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