My buddy didn't do his first spark plug change until 170,000 miles on his '04 Laredo. He thought he had done it back at 100K...
Had to get a smog check, and noticed the idle was a bit rougher than it should be. Pulled the plugs and the center electrodes were eroded almost down to the porcelain. lol
The ignition was jumping about an 0.080" gap.
Along with being smoother at idle, he noticed a bit better fuel mileage with the new plugs.
The funny part is, he regularly pulls a heavy trailer behind the Jeep. He's over 240K miles now, and still running strong.
Some people have all the luck. :elkgrin: