The denominator changes as well
Velocity is a ratio of a spatial difference to a time difference. Transform both differences before dividing, and light speed stays c while collinear subluminal speeds compose below c.
Read · Special relativity: from clock operations to electrodynamics · Results
§5 · velocity composition
Follow the introduction and all ten sections, including field transformations, finite light complexes, moving mirrors, charge-current transformations, and the electron-force conventions.
Newly authored explanatory preview in modern notation, with editorial and physics review pending. Both the kinematic and electrodynamic halves are treated, but this is not a German transcription, an aligned translation, or a complete critical edition. The source paragraphs, footnotes, acknowledgment, and date-lines are not claimed to be fully represented or reviewed here. Headings and argument units are editorial.