Their site states that a “4 ohm speaker” will measure at about 3.2 ohms with a multimeter, so that tells me that you want a “4 ohm speaker” which is pretty common in automotive applications (while home stereos use 8 ohm speakers).
But the devil is in the details. you don’t want to burn out your output transistors. http://www.turnswitch.com/speakers.htm has some selections so you could call them
I want (need) to replace the dash speaker in my 59 Dodge
(5x7), but need to know what ohm the AM radio will support. Appears most
modern speakers are much higher ohms than the older cars and I'm not up-to-speed
on how that works.