Based on the vehicles I have had in the past few years I noticed that, enven though they are different brands, they have identical MPH-to-RPM measures at high speed. We have two 2000 Lexus RX-300 (by wife and daughter), one 1995 Nissan Quest (my work horse and family van), and one 1987 MB 420SEL (my restoration project) in the house. All automatic with overdrive.

They all hit 70 MPH (determined by GPS) at RPM (from the RPM reading in each car) in the exact same lag of travel on the same route of 30 miles. The test is done by using the cruise control and readings were taken whenever the road is flat. I recall my wife's previous 1988 Toyota Cressida had the same reading. The gears were in D with overdrive enabled in the RX300 and Quest, while the 420SEL was in D (no explicit OD button).

My question is how come these cars by different manufactures converge on a design point so close (if not exact) in terms of spead-rpm? Maybe the same engineering equation(s) is used by the industry?