340 Gets Terrible Gas Mileage

I put NGK copper-core in everything. In my experience wrenching (as a hobby) for about 14 years now I've never had a single one fail, and every one I pull out of the box looks perfectly made. And I've installed total probably about 20-30 sets in various engines, mostly cars and trucks. Super consistent and quality manufactured, they're made in Japan after all and NGK is a long-time sponsor of Formula 1 racing and supplies spark plugs and ignition parts to most (if not all) of those teams. I ran a set of NGKs for well over 10k miles (not really intentionally) in a warmed-over carb'd 360 and no failures or foul-outs. I only ever have to replace them as a time/mileage thing as part of usual maintenance. I'm sure in other conditions it might be different and I haven't yet tried them in a 2-stroke, aside from I think my parents' weed trimmer lol.

I ran a set of Autolite coppers in the 4.0L I-6 in my old Jeep, after about 2 years they had a bunch of surface rust on the steel hex section and the center electrodes had all seriously eroded to being almost non-existent. That brand is owned by FRAM which is all I need to know...