I don't understand why single ground makes the truck run like crap. I mean, I get why they wouldn't last as long, but when the plug is brand new and still working properly why is that a problem? It's a very simple thing, it's job is to produce a spark when current is applied. So are brand new single ground plugs actually failing to produce spark, and if so why?
And there are no dual ground Iridium plugs AFAIK, yet people are using those with success.
I'm sure somebody with an EE can offer a more eloquent response, but I'll take a shot at it.
A spark plug firing twice as often will wear down more than twice as fast. That's because erosion will occur faster the hotter the elements are, and they'll stay hotter longer due to the 2x firing cycle. So a copper plug rated for 25,000 miles in a waste spark system may start to run poorly after 8-10,000 miles, rather than 12,500.
This is, incidentally, why copper and, to a lesser extent, platinum plugs need to be gapped, while iridium do not. You're supposed to gap copper such that there is sufficient heat generated by the resistance between the electrode and ground to ignite the gas in the cylinder over the lifespan of the plug, even as it wears down. This is why different gaps are specified -- depending on the vehicle's compression ratio and a few other factors, a copper plug that will ignite poorly in one vehicle may do fine in another. Iridium and platinum are designed to wear much more slowly, making gapping less critical for them.
Then, there's the matter of the actual surface of the plugs. Copper can pit more readily, and will also experience smaller pits of erosion on its surface. This will result in the plug itself experience a small but non-negligible amount of internal resistance before it even generates a spark.
Finally, there's the matter of QA. Every manufacturer specifies tolerances. Platinum and iridium plugs are more expensive, thus generating higher margins; it's in the manufacturer's interest to ensure they don't get returned for warranty or other problems.
People that spend less than a buck a plug, however, would generally fall into a, ahem, more cost-conscious demographic that probably would not be able to notice the poorer performance of said plug in their typical application. So, if you buy 100 copper plugs and 100 platinum plugs, I can virtually guarantee you that there will be one or two more out-of-spec plugs in the copper lot than there will be in the platinum lot.
