How long before I should install new spark plugs? My 2001 GT has 62,000 miles on it, and the plugs have never been checked or replaced. The car seems to run just fine. I am a spirited driver, but I don't drag race the car or often take it over 4,000 rpm. I use regular (87 octane) gas as recommended, and the only mods I have are a K&N air filter and a Spintech catted x-pipe with a Spintech catback.