Grab a multimeter and check the resistance of a whole bunch of different speakers. That will give you a feeling of whats going on and go from there.
The measurements will vary but, will give you an idea of what to expect in the real world.
If you were wiring a series/parallel setup, use the manufacturers spec and calculate from there. When finished wiring them up, read the resistance across the final load to see if you are in an accepatble range. If your close to the expected resistance, then you should be alright given your amplifier can handle that load.
Impedance is dynamic in loudspeakers. Can, and does, vary across the frequency spectrum.
You can understand alot by using a multimeter for everything you do electrically.
I too came from the car audio industry. Attended Rockford Fosgate's 3 day course they offered back in the late '80's in Tempe AZ. Still have the 3 ring binder somewhere in my shop.
It seems that car audio guys,many times, have a better understanding of system design from experience. (Breaking shit)