I can attest that mileage for 80+mph driving still seems to be in the 47-48mpg imperial range.
That, or the onboard calculator is totally fukked.
I had a 200km drive to make (400 actually both ways) and I drove fast on the way to Barrie to make an apptmt. I thought I could do a 55+mpg slow run home to make up for the dash. So, I drove about 80km/h like an old lady on our crazy freeway coming home. The best the indicator would rise to was 16.5km/l . Didn't matter if I was drafting, coasting down hills, whatever. It was stuck at 16.5km/l. With nothing to lose except money I decided to drive faster to fix it. Driving 110kmh for 20 kms didn't help so I just thought what the hell, I'll keep up with the boy racers (about 80-85mph or so).
To my surprise! and shock the rate rose a bit to 16.9km/l!! I thought, are you kidding me, I've been going 85-90mph for the last hour and now you tell me my mileage is improving? BTW, that's about 4,000+ rpm and way out of the eco zone.
FWIW I filled up at the end of the crazy run to see I my mileage was 47.5 imperial or so. Now I'm wondering if there's any point driving below the limit to achieve high or super high mileage. Might as well just go for it.
I don't have any fear of engine wear. My 400cc Burgmans run 7-8000rpm all day long pulling me and a passenger on hot hot days. And there are Burgman engines out there with 150,000 miles on them.
I did get blown around a lot @90mph I'll admit. But so did other cars I noticed.