friend fixing to use 18 gauge sub wiring

He's fine. It is minimal SPL difference. Even though 12ga would be preferable. It depends if it is ACTUALLY 18ga wire or "labeled" as 18ga but actually 22 ga.

Was he planning on doubling it up to "increase the gauge" too or just running a single wire?

No just regular single wire.2003 Modded Mustang GT

If the speaker wire run is short (a few feet), it should work. The longer the wire, the bigger the impact. It can show up in SPL as mentioned before. Then again, people will do what they do no matter what. Threaten him that it could heat up his equipment too much due to resistance and it will all melt (figure the cost of 12-14 ga versus new amp/sub). Not that it would work (wink-wink) on him anyway........

Will do. I'm running 12 guage sub wiring and only have about 2 feet from the box to amp. I almost think I wasted my money now, lol.2003 Modded Mustang GT

Nah, you're just maximizing the power transfer and eliminating a possible SPL reduction. I run 16 ga (real 16 ga that is) for 300w RMS. But the wire is 4 feet. If that changed (more watts, longer run), I would consider 14 g. So far it hasn't been an issue.

800WRMS through 20 feet of 18g wire into a 2 ohm load equals a loss of 1.04dB. Anything less than 20 feet, and he'll never hear the loss, period. He'll be fine.It all reminds me of something that Molière once said to Guy de Maupassant at a café in Vienna: "That's nice. You should write it down."