"** Got the tiniest idea what happens if you never clean the pinch roller??

Obviously not. "

The worst part is that the pinch roller is on the oxide side of the tape. Older VCRs were like that but they fixed it by using an elevator gear and dropping the pinch roller to the inside. There is no practical way to do this with an audio cassette.

Actually eight tracks were better in that respect, though the pinch roller was included in the cassette. Actually, some of them did not use rubber ones, they just made them out of plastic and they played fine. Actually they could outperform cassettes as they had double the tape speed. But there were other limitations, like no rewind.

To clean a pinch roller I usually used something abrasive, like a paint scuffing pad. Like the idler wheel in a turntable, if you make it a bit smaller it does not change the speed. In the case of a turntable it is the diameter of the motor shaft, in a tape deck it is a matter of the capstan diameter. Just don't make it small enough that it does not engage.

Oe main thing to keep the wow and flutter down is to make it even. If it is uneven you got problems. I almost always had the deck running when I cleaned the pinch roller, let it turn. I did the same with VCRs by using a dummy tape.

Anyway, older car cassette decks can be a real PITA. You might just want to get one of those fake cassettes that you plug something in to, like an MP3 player or something. Very few car cassette players were better than a 160 K MP3, if any. Or there are those FM modulators that just transmit to the FM, nut those are limited to 15 KHz. But do you really care when you are driving across Texas ? I doubt it.