I remember the Dynaflow transmissions Buicks had. You started out in high [direct drive] and never felt any shifting as you got to highway speed.
And my 1965 Riviera, which got 9 mpg in town and 13 on the highway.
Then he bought a gorgeous Toronado...midnight blue. I used to see the mechanic driving it around all the time!
Actually, you started out in "low"-- the engine speed was what was high.
The classic Dynaflow was pure fluid drive; as you say, no gears. I was fascinated by a clear plastic scale model at our Buick dealers's. It sat on a little wood base and you could crank it to watch it operate. (Dad became a Buick Man in '51 and never looked back.)
Problem with Dynaflow was efficency; i.e., gas mileage. In the 60's, GM standardized on the Turbo-Hydramatic, originally an Oldsmobile design, so Buick switched to that.
Being a more traditional design, the Hydramatic had a multiple-ratio gear train (3 or 4 speeds) driven by a "torque converter," a fluid turbine that evened out the torque discontinuities at startup and during gear changes.
In contrast, the Dynaflow was just a big torque converter with no variable gearing downstream. Late designs had variable pitch blades in the turbine.
The torque converter has the interesting virtue of increasing its effective step-down ratio when the torque load on it increases, and it does this with no meshing parts; just the impeller, the turbine, and the fluid. This makes it a good complement to the hard gear ratios downstream in the main body of the transmission. And, of course, it tolerates being stalled; i.e., having the engine idle while you are stopped but still in gear. In essence, it is the equivalent of the clutch in a manual transmission.
Now the torque converter is lossy, for the same reason that the old Dynaflow was: losses in the fluid flow. Modern automatic transmissions "lock up" the torque converter during the long periods when you're just cruising along at a constant speed, making little torque demand on the engine.