it is a slightly colder plug, but the plugs at 104 level are more of a enhancement, even on the stock pulley. they're supporting mods, you don't "gain" horsepower with them, you just make the car run better. honestly i've never heard of losing mileage with the plugs, only if you go to a lower thermostat. if you don't go with 104s, and you do stockers, again, don't bother with Iridium or any crap like that. waste of money. you should be fine running those plugs though.