I do an 84 mile round trip each day in my 220d, 80% motorway and average 35-36mpg. I cant use 6th as anything less than 90 will have you dropping to 5th every five minutes when you dab the brakes! However, there doesn't seem to be many guys with the 220d sport on here complaining, so maybe the different gearbox improves things?
TBH with the mileage you're quoting a petrol would make better sense.
Then again don't listen to me I'm just on here to moan at Lexus GB!