This was mentioned in the article as the main reason for the legacy 55mph limit imposed after the Saudi oil embargo.

Oops, I see that now. My fault for only reading the first section of the article.

That number comes from a time of 3-speed transmissions and non-aerodynamic bodies. I heard better numbers back when 5-speed was becoming common, but now we're seeing 8-speed and 9-speed and CVT. It's probably around 65mph now.

I've heard 70km/h elsewhere, but the only personal data I have is I used to drive a 2002 5-speed manual VW Golf Mk IV, the 2L petrol version, and that had a fuel economy display on it plus a nice chart of economy per gear in the owner's manual. The book similarly said best would be at around 70km/h (~45mph) in 5th gear, and I confirmed that was true in my own tests. The 6-speed GTI was charted in the owner's manual as well and its best economy was shown as a little faster, in its extra 6th gear.

Aerodynamic resistance (“drag”) doesn't care about your transmission. The sweet spot is even lower with electric cars (especially with all the peripheral devices turned off).

However, a modern mid-sized car with an efficient engine gets 40 MPG or more at 75 MPH, while a gas guzzler won't get such mileage at any speed.

Own a 2014 6-speed petrol VW and according to the manual the optimal speed for max mileage is around 70-75 km/h

Spark ignition engines suffer from pumping losses when throttled. So it is a pretty broad generalization to throw that number out. Fuel economy will depend on rolling resistance, gearing, and aerodynamics.