Mine's an auto and it doesn't make any difference whatsoever on my car! Quite strangely, I've fitted slightly oversize tyres and on the journey to my in-laws (approx 135 miles), it's still 22 mpg! How wierd is that?
That's about right on a run, the last time I checked my disco's mpg was around 4 years ago on a run to Devon via M25, M3 and the infamous A303 I got 22 mpg. On the way home via M5, M4, M25 I got 18 mpg and I was home in half the time to going... for me motorway speed is as fast as I can safely get a way with.
My old 4.2 Maserati Quattroporte, now part ex for a V6 Jaguar, did 36 mpg on a light throttle at 70mph and 20 in traffic according to the dash display, so not all V8s are juicey, although it was single figures when I went on a motorway, I never did reach its max of 177mph
Would that be an individual V8 or V8s as a whole. individually I've never noticed, but J Clarkson when on Top Gear took an Aston Martin to Switzerland said he used more fuel fuel with CC engaged, although that car was a V12.