Every cassette deck runs at a slightly different speed. To calibrate your machine you need a test tape and a frequency counter.

Here’s a BASF 3150 HZ Wow and Flutter Control test tape which is also suitable for speed calibration. It’s hooked up to a Fluke 45 digital multimeter. The meter is reading back 3.1502 kHz which is as close as possible to the reference frequency. On these direct-drive decks the speed may drift 0.1% to 0.3% over 500 hours of running time. On belt-drive and consumer cassette decks you’ll probably see a much higher speed variance in a much shorter time.