I recently had a road trip from Columbus up to Cleveland. I was by myself, bored and traffic was light. I started driving in 10 mile intervals at 70, reset my mpg meter, and then would drive 10 miles at 75. Repeated this 5 times. Results below!
Run 70mph 75mph
1 29.2 27.6
2 29.5 28.5
3 29.7 30.2
4 29.8 29.2
5 29.8 28.1
avg 29.6 28.7

Some note: It was between 75-80 degrees. Wind seems light-medium and as much left-to-right as it was a tailwind. The area is "rolling hills" but nothing severe, but not flat either. Driving a '14 SH-AWD with 6-speed manual transmission. ~45,000 mi. I also had a new clutch and flywheel put in recently and noticed about a 1.5 mpg increase per tank.

I was mildly surprised that there was so little spread between the two speeds. The 30.2 number felt like I got "lucky" and had a good stretch of road with a little more downhill than uphill, plus was going more with the wind, than having it blow from the side. I feel like the meter in the TL is the most accurate MPG meter I've ever used.

TL SH-AWD 6MT (Acura TL) | Fuelly

 

I got about the same as well in L/100. The best I could do was 8.1 at about 110-120 km/h. I love using cruise control. That helps a lot too.

 

Try driving a 6,600lbs truck with a brick wall for a grille. I can almost visibly see the gas gauge dip when I drive 75.

These cars are designed to offer a smooth slice through the wind. You wouldn't notice much difference in CD from 60-80.

 

I know the EPA tests don't reflect it, but based on our two cars, my TL AWD and my wife's Accord EX 4 cylinder (both with 6MTs), I believe that the MT in both cars return much better mileage than the EPA official numbers portray. Our Accord is rated 34 mpg on the highway (2 less than the CVT 4 cylinder), but we can routinely achieve over 40 mpg when cruising the highway in non-winter conditions. My TL routinely runs 28-30 when doing highway cruising.

 

20 miles would probably be a better interval to smooth out any hills or passing you might have done while driving. No surprise that faster was worse mpg.
I see no reason why a clutch/flywheel would have any influence unless there was a serious problem with the old equipment.

 

Pretty sure the 6AT is more fuel efficient than the 6MT.
I don't think there's any 6AT+ tranny from any manufacturer that has worse mpg than MT in the same car anymore in normal driving conditions.
But you can always nurse a MT to do better than an AT by shifting before 2k rpm, but you may as well be riding an electric bike then!