A properly engineered tape format will have, for lack of a better name, an "index" or synchronisation bit or signal recorded alongside the data tracks. Back in the days of videotape there was something called "control track" that served a similar purpose. In the video case it was used mostly to ensure the video being read off tape ran as close as possible to the required video standard frequency. Smaller mechanical variations where then removed electronically using a FIFO often called "time base corrector". Similar approaches can (and should) be used for data tapes in order to ensure data integrity in the face of tape stretch, etc. The other approach is to encode the data using a self clocking encoder, in which case data recovery is pretty much guaranteed at any speed so long as the data read frequency is within the read circuitry VCO's capture range. These approaches are not mutually exclusive. I think there have been hybrid optical/magnetic approches also but I am not familiar with them.