Unfortunately for my Bronco (not so bad in the mid-seventies with the 55mph speed though) the sweet-spot is between 55 and 60. With 55 being consistently better.
My '89 Caddy (not too aero either, but not bad) with 4.5 V8 and freeway gears would get a full 29-30 mpg, but only at between 60 and 63 max. 28 if the ground wasn't perfectly level.
Toyota Corolla, best at 60. '95 Mustang, best at 60. F150 with 302 and automatic w/2bbl and computer controlled ignition w/3.5 gears, got 12 under any condition. No power under any conditions either. Toyota pickup, crappy mileage no matter what!
The old Buick, now it could get into the 23 mpg range with it's old 430ci V8 with Quadrajet and total freeway-flyer gears (something in the high 2's!) at between 70 and 75.
My old 4-carb Corvair (VERY aero) was humming along at a 3750 rpm with it's low 3.55 gears and tiny tires, but got it's best mileage (30 to 32 mpg) at 70 mph, but only when going uphill, like to Tahoe or semi-uphill like to SF. Otherwise it's max was 26 to 28 mpg.
The only time my F350 with 400 2bbl got a whopping 13.3 mpg was at 55 with a full load of crap, where the truck weighed in at 8850 lbs. on level ground at a super steady speed between gas stations on the way to Oregon. On the way back, empty, at the same speed I reverted to the normal 10 to 11 mpg. Even when I filled up after the long downhill sections near the border. Downhill!
Every vehicle seems to be different, but they also seems to mostly follow the rules about less aero vehicles prefer less speed, and some carbureted vehicles actually get better mileage with the throttle pushed farther open for a given speed (uphill and/or loaded). EFI depends on the tuner and what they knew of course, but has some similarities to the carbs sometimes.
All my carbureted cars got better mileage at higher rpms and speeds than at lower. All my carbureted trucks got their best mileage at lower speeds and higher throttle openings.
My '03 Buick 3.8 V6 gets a consistent 28-29 at any speed between 55 and 70. But will get 31 when loaded with 5 people and all their gear for a week. Go figure. There's that higher throttle opening again maybe? Lower ride height perhaps? Dunno, but I like it.
All this is over years of checking almost every tank of gas, religiously, due to either business needs or just for fun.
Paul