Agree somewhat w/OlDirtyJohn wrt no Performance gains from sparkplugs if your talking HP or torque. But since I switched to NGK Iridium (2315), my tailpipes are just about as clean as a whistle with minimal soot. When I ran the coppers I had to scrub my tailpipes frequently with SOS to get them clean. So in my case I feel the Iridiums perform better, (and in my world believe "leaner burn") but I also put cam and headers in when I changed to them so I cannot truly comment whether Iridiums would improve a stock truck mileage numbers. These plugs cost me twice what they sell for in the US ($225 Canadian) but I think they were worth it. Now some may ask why did I go 2 Temp colder than stock well that decision was based on alot of research / forum reads / and what others where running with certain mods and the fact Superchips said they did not support my mods when I bought it but since my old 3714 worked fine they agreed I would be OK with the 3865 but no guarantee. I must admit that my best mileage ever was when my truck was stock with new Champion Copper plugs (soot aside), plug wires and new air filter, and stock tires (265/70R17) with PSI as per P245 door tag. I got a best 17 MPG (Little Bigger Gallons 4.5L though) on the HWY for a full tank on roads with 55 to 65 MPH speedlimit. IMO I would recommend 1st NGK Traditional Copper Type or go Iridium. At Moes Performance Site there is a chart description which explains the Hemi heat ranges and applications. If you are going to do performance mods you may want to get a head start with one heat range colder. My original choice was the NGK 4306 copper but was told they are not listed in NGK Canada so I dug deep and bought the expensive ones that will last twice as long. Apoligize for long post. Hope the Info helps.