I haven't watched numbers on many of my cars, but my Jetta SportWagen TDI had peak mpg right around 60 mph, at least it did when I had steered the wheel at all since startup ;). The gear ratios are setup to deliver peak efficiency at highway speeds, although you do hit a point where the v squared in drag wins. EV is different, because motors are strictly worse the faster they spin.
Vehicle provided instant MPG data is always a little suspect, of course.
Ah so that is kind of the point I wanted to try and make.
The engine isn't any less efficient in lower gears. It's just that you are measuring MPG and not some other metric like hours per gallon.
If you are able to travel at 60 the entire time then sure, but at the same RPM your engine is consuming the same fuel in a lower gear.
I'm pretty sure that MPG value would also very aggressively change if you were constantly accelerating and deceleration to hit that 60 MPH number whereas it would in fact have been more efficient to travel at a lower constant speed.
Vehicle provided instant MPG data is always a little suspect, of course.