In article -
september.org, says...

Oh, smarter-than-I people,

http://imgur.com/WPEoOu1

The 2 belt-drive capstan flywheels in an auto-reverse cassette mechanism are
different sizes. (Motor drive pulley will be in the small loop end of the
belt in illustration above.)

How does that result in the same tape speed in both directions?

Confused...

Its been a while since I've seen the inside of a cassett drive.

But memory tells me you have a pinch roller with pin shaft that governs
the actual speed.

Jamie