I ran my first set of plugs 60,000 miles and the second set for 75,000 miles. Both times, I never could tell ANY difference in performance or gas mileage when the plugs were replaced. I'm going to try to get 100,000 miles out of this last set, if I even keep the car that long.

-Mark- '99 Mystique LS, totally original (including the original water pump and the premium stereo)! Bought new in December 1998 for $21,000. 208,000 miles