[Richard]

Originally Posted by Rust_never_sleeps

I've been toying with using a garage sale 4Ω speaker to replace my crispy old 10Ω AC/Delco unit.
Came across this, hope it's useful:

https://soundcertified.com/how-to-ad...tch-impedance/

The problem with this is mentioned in the link you provided. Diminished volume. In your case, bringing a 4 ohm speaker up to 10 ohms. About 60% of the output from the stereo will be dissipated by the resistor. The stock AM/FM in my 72 is not very loud at full volume.