So I bought my wife, girlfriend at the time a used Jeep Liberty in 2007. It was a 2003 model with roughly 44k miles on it. I just changed the spark plugs yesterday (at 93k miles) for the 1st time, thinking they were platinum I thought I would change them every 100k.

To my surprise I found copper NGK-R's (copper) I checked the gap on them and they were at .080. Could this be why it gets horrible gas mileage? (16 mpg). How long should I keep the platinum's in there? (the are NGK-G power platinum)

 

heh... yah, so, Jeep uses copper plugs in almost everything...

I would say probably 60k on the platinums... 60k is pretty standard on the platinum plugs... iridiums are good to 100k usually...

in comparison to fuel mileage and stuff, its really not that expensive to change the plugs a little more often... I used to do copper plugs every 15k miles in the Lexus and more often in my turbo Talon...

 

best bet is to go to the dealer and get the OEM plugs. i have found over the years certain cars like certain plugs.
regardless of price, the OEM plugs always work well, just becuase you put fancy plugs in a motor doesnt mean it will run better.

on another note, if you want the correct gapping, just look at the sticker on the inside of the hood.