I have got in trouble on forums for writing this, but if you take an engineering class on automotive design, in the sections on transmissions and gearing you will be taught that engineering convention years ago (1930s) developed the term “overdrive ratio” and it did not originally mean a gear or separate unit like a Gearvendors.

Engineers originally had the straightforward thought that:
“Top gear should be set to let the maximum horsepower of the engine get the vehicle up to its top speed on level highways”.

If this was set correctly, the “overdrive ratio” was 1.00
regardless whether the driveshaft was turing at a lower rpm than the engine.

If the the vehicle did not reach its theoretical top speed and its engine was BELOW the rpm of maximum horsepower, then the “overdrive ratio” was less than 1.00 and maybe 0.90.

If the vehicle did not reach its top speed and the engine rpm was ABOVE its rpm of maximum horsepower, the the “overdrive ratio” was a number above 1.00, say 1.1

Books like the Bosch Automotive Handbook have a German engineering slant and still teach it this way in their section on transmissions.

Yes, it is more “word definitions” but one ought to respect the past traditions.
When Watt defined a “horsepower” for his steam engine it became a tradition even if that amount of power would quickly kill even a Cyldesdale sized horse.