Quote: I have ran both the AR 3911 and the NGK 7173 both extended tip only change was plug in my case car ran same et but with the AR 3911 I gained 1 mph and the car idles much better this was back to back test same day 30 minutes apart.
This was replacing and old set with new AR's & new NGK's? or were you replacing the NGK's with AR's?
