i could tell right away, my throttle was more responsive, and it ran smoother. now with that in mind, this is not any kind of huge thing. that was after i had dual exaust and before i added the cai and tune. i was in the 50,000 range on miles and you should replace your wires around 60,000 under normal maintenance anyway. good time to upgrade. with all the mods i have that claim improved milage, i havent noticed any more than 1 maybe 2 in mpg gains at best. but i havent had a chance to take a long trip with the 91 performance tune yet.