Fine wire plugs are the new standard in Ford waste fire systems. (for that matter many companies are using them now)
Why is that?
Because the amount of current required to start the spark jumping is exponentially less with a fine wire plug there is not the dramatic transfer of material every time.
Also the platinum used in the fine wire plugs is significantly better quality.
You will get "roughly" the same mileage out of a "properly" designed fine wire plug as a double platinum plug. 40-60k tops.
Fine wire plugs have better throttle response and promote less emissions. (stronger, more efficient spark kernel)
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To respond to the original question...
Why are you running a step colder plug?
You are loosing mpg and even a little performance doing this on any car but a high compression hybrid with a wicked timing curve that is also running race fuel. (that or boost)
I found no benefit to running 1 step colder plugs on my 10.3 CR SVT with ignition timing near double the stock curves. I did lose mpg from running them. (i.e. they were too cold for optimum non-WOT fuel burn)
