Originally Posted by gts007
I changed my plugs for the 1st time at 120,000+ lol. Noticed no difference in power or anything like that but my mileage definitely shot up
Yeah, the plugs will wear, and you won't really notice any drivability differences because the computer seems to compensate and keep everything running reasonably well, even as the plugs are wearing down.
Recommended interval in the owner's manual for my '03 4,0 liter is 100,000 kilometers (~ 60,000 miles) and that's what I go by - seems to be a reasonable interval with platinum plugs but you can change out earlier if you like - better sooner than later, and better to change them before you encounter problems with them.
Wires you can probably stretch out a little longer before you have to change them, but I just change them out when I change plugs.