25 years ago I had access to a professional audio-visual studio and managed
to record a couple of tapes with crystal derived 1KHz sine tones on a
calibrated cassette deck. Since then I've mangled one and only one remains
so reluctant to use it, maybe stretched for all I know, no accesss to known
good player these days.

Decided instead to use a strobe LED and f-counter coupled sig gen shone
directly on the capstan (x4 speed for easier use , seeing the strobing image
at x1 LED pulsing is very difficult). Also now knowing that spindles come in
sizes like 1.99mm ... 2.49 mm never seemingly 1.95mm , always ending with a
9 so no great accuracy required to measure the spindles , often with
restricted access for mic/calipers
Then maths for tape speed for 1 7/8 inches per second.
This worked fine for 2.29mm spindle and for a hyper-critical musician.

I repeat this technique this week for a much more basic machine which has a
1.69mm spindle and scaling for that, adjusting motor speed and even I could
tell ,playing a familiar music tape ,that the tempo was wrong.
Placing 1KHz test tape in and checking beats with a f-counter coupled sig
gen and it was 1 and 1/3 semitones too fast. So what have I not taken into
account in the linear scaling, influence of tape thickness ? tape curl
around a tighter spindle? different rubber formulation of the pinch wheel?

In the same vein what is the piece of Wagner music that has something like
39 bars of the same note played from the beginning , Tristan & Isolde ? .
What other piece of music commercially recorded onto cassette tape, to look
out for, containing a sustained single note it might be possible to use as a
test tape. ?

Anyone use a tape with any old constant tone recorded on it but small
magnet
erased start and stop points of 100 seconds say by measured length of 1.875
x 100 inches ( by pulling out of the cassette and then rewinding back in )?
Would be slow but would be a last back stop I suppose for cross-comparison