I thought I understood the basics of speaker wiring, parallel vs series and I was hooking up my speakers, but math is not agreeing with reality. I have two 8 ohm speakers and one 4 ohm. I wired the two 8 ohm speakers in parallel, then tied those to the 4 ohm in series (notice the sign change in the graphic). I expected the total impedance to be 8 ohms, but I measure 2 ohms. What am I missing?

All three are in parallel but the 4 ohm speaker is out of phase. You would need to run the first 4 ohm speaker and the 2nd middle 8 0hm in series.

From jack positive to "4 ohm" positive. From 4 ohm negative speaker to 8 ohm positive (middle speaker. From 8 ohm negative (middle speaker) to jack negative.

The jack would be on the speaker cab jack that you would connect your amp speaker out to the cab input ( or jack).

you've just got to remember, that to have series, you need a loop, as illustrated on the right ½ of my drawing. To be parallel, all +'s are connected together & all -'s together. The loop goes + to -, to + to - parallel: +/+/+ -/-/- series +/-/+/- here, you have a mix, of series, w/ 2 of the speakers hooked up parallel.

I wanted to close out this thread with the rest of the story. I have a Valvestate S80 (8240) combo and a stereo 412 cab I was putting together. The S80 did not come with speaker out jacks only DI jacks. I used the wiring in this thread and all works good. The combo and cab are wired as a stereo system. Very happy with my new practice amp.