The automatic transmission is continuously adjustable

stepless

Examples:

Stepless entrances, elevators, disabled toilets and barrier-free lecture and seminar rooms [...] are still not found everywhere. [Die Zeit, 14.06.2012]

This includes building entrances with no steps, stairs with several spacious platforms, etc. [Neue Zürcher Zeitung, 25.11.2003]

Inside the tower, there were 2 overlapping, stepless ramps over a circular floor plan, on which donkeys carried the oil up to the light source. [Olbrich, Harald (ed.): Lexikon der Kunst. Berlin: Directmedia Publ. 2001 [1992], p. 18394]

The ground floor must be accessible without steps, elevator with space for a wheelchair, ramps with a maximum gradient of 6%, […] [Klee, Ernst: Behinderten-Report, Frankfurt a. M .: Fischer Taschenbuch-Verl. 1981 [1974], p. 67]

[...] I had inspected it [the tower] during the day and realized that it was impossible for a strong man to climb this rotten, almost stepless, sky-high ladder. [Arnim, Bettina von: Goethe’s correspondence with a child. Vol. 1. Berlin, 1835]

transmitted without recognizable intermediate stages, seamlessly

Collocations:

as an adjective attribute: continuously variable automatic transmission; smooth transition

as adverbial definition: continuously adjustable; to regulate sth

Examples:

This [the focal length] is usually continuously changed by turning the lens. [Which lens is the right one for a trip ?, May 27, 2015, accessed on February 15, 2017]

Tom Hiddleston [...] plays this role as if he had a built-in, stepless regulation for degrees of sophistication. [Die Zeit, 23.06.2016]

The sticks for children can be adjusted continuously from 72 to 120 centimeters, the handle is adapted to children's hands. [Süddeutsche Zeitung, 18.06.2012]

The continuously variable automatic transmission, which can optionally also be operated manually, will be used for the first time […]. [Neue Zürcher Zeitung, 25.08.2003]

The glow of reed grass in the moonlight [...] gradually fades out in the black of the picture background, as if the blowing vegetation disappeared in the mild moonlit night. [Neue Zürcher Zeitung, 25.07.2002]