Interesting. We have a tall windshield too, but I can't say we reach 87mph in a few seconds without noticing. In fact, I could never exceed 84 on level road at all (we often ride 2-up and the two of us combined weigh just a tad more than your dad so we may be a good reference here
) - but the gas mileage is mostly in the 70s lately as you can see it in my log (Ciliegia is the GV250, now we're after the 10000 km maint, carbs tuned).
It's very mixed usage, contains backroads, main roads, freeway over 70mph and city learning (my girlfriend was pretty upset after a lesson because her partner was driving under 20mph on main streets where 37 is the legal limit).
It's said that top speed can get better with 'longer' gearing and I expect FE to improve too - it may be our next experiment (new chain+sprockets at 15k maint like this summer?).