i have over 75k miles on my NGK tr55-IX iridum plugs.
i pulled a plug at 60k and the gap had grown by .004.
no big deal , i gaped my plugs at .050 to start with.
car gets 27 to 30 mpg at 70 to 75 mph.
i have 108k miles on the car and it still runs great.
you can see my mods below.