If you want it simple to start, I don't think iridium spark plugs effect the car's mpg entirely. My first experience with the iridium spark plug in my car was that it moved a lot faster, that the engine stayed tuned more easily, and the easier it is to have an engine running smooth and easy, the more efficient it is, but technically I can have that result with platinum spark plugs, as far as the tuning goes, but it's much more effort requires re-gapping the spark plugs frequently. So, as a whole, the iridium spark plug will get you better mpg for your car, but not necessarily on the short run. The iridium tip is much smaller than the standard spark plug for the 1990's, which it allows it to get to the self-cleaning temperature faster, prevents spark plug fowling, and allows for the lightning that a spark plug makes to be at the appropriate spark plug gap better than if it was wider. You see, as the materials in a spark plug corrode or erode over time it'll make the lightning go to the side of a plug, rather than the center, and this can throw off the actual distance the spark is going versus what your spark plug gapping tool can measure. This is one of the reasons replacing plugs that work good will save you money, and this is another reason the iridium spark plug will last longer. To learn more about iridium spark plugs, go to my NGK Spark Plug Advantage page.
Update 9 months later: I would have had to replace these iridium spark plugs if they had the longevity of platinum's by now. Not that the platinum spark plugs wouldn't have kept working, but they'd be effecting my car's mpg negatively.
Note: I had the iridium spark plugs in my car since the summer of 2009. It's just now time to check their gap, let alone the fact that I'd usually would have had to replace them by now. Be careful when adjusting and checking gaps on iridium's! The iridium tip for the electrode breaks off easily! When adjusting the prong above them, don't pry against the electrode. Use pliers and just touch the prong that reaches up and over from it's sides. See my 3 spark plug gap articles via this word link spark plug gap.
How about some legitimate hands on for spark plug and wire balancing technique!
