It depends on your driving style and what type of driving. Highway driving is going to be less damaging to plugs because the engine won’t be firing as much. 75k of all city driving and those plugs are probably on their last legs.
A friend of mine was troubleshooting the “strange vibration at idle” on his 2003 Toyota Echo last year.
That car is something like 250K+ miles already.
Knowing he does not believe in any maintenance other than oil change, I raise to him that probably it’s time to replace spark plugs and make catch up on other things.
He dismissed that suggestion with a healthy doze of sarcasm, and went on various other routes like accessories on the serpentine belt, engine mounts and such.
Eventually, he dug down to compression measurements, where he needed to remove the plugs and then he sent me a picture over SMS with the plug side electrode completely eroded away.
Likely, the plug was firing against the piston, yet the engine was not yet giving any CEL light or codes.
Indeed, Toyota makes quite reliable cars, I thought 
Upon the plug replacement, the vibration was gone… Go figure?
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Yeh I too know a few people who use a parts gun when their vehicle has a problem. Invariably they start with the most expensive parts first.
Not so much these days because the coils sit on the plugs but a lumpy, stumbling idle, hard starting especially if it’s very humid and/or a high speed misfire = generally bad plugs.
I had a '70 Opel GT where I changed the plugs, condenser and points when I changed the oil, 3000 mi.
I thought all spark plugs were the same, until the 2008 Ford F-150 came along. I bought it used with 120,000 miles and the dealer tells me it’s time for spark plugs. I say, “Don’t change them, I’ll just do it on my own,” as I have changed many. Then he says, “These are different… and we will charge you $500 to change them.” I disregard the dealer and go on my way.
Knowing that I am 1000% percent confident, (Yes, that is a thousand. I have been right soooo many times) I ask dealer #2, a friend with the same model, and an old auto rabbi. They all say the same thing, and dealer #2 says, “If the plugs have not been changed before then they may break off, and we will have to charge you an extra $60 dollars and add time to the repair.”
I cave in… and go back to dealer #1 and plead guilty by turning myself in. They comfort me in this trying time and tell me the procedure which will have to take place. I can decrease the risk of a “broke off spark plug,” by adding a $100 injector cleaning process, that bypasses the fuel and burns a special blend of cleaner for an hour or so. Then they will run the engine to operating temperature and remove and replace one plug, followed buy running up to temperature and replace a 2nd plug, and so on until all eight are replaced.
I think it cost $400 dollars and they didn’t have a plug break off. The plugs looked like they had been replaced before, so that helped.
I asked to see a plug and keep for story telling, which has given me the pleasure of telling the story about 5 times now. In closing, hope the Mazda engineers were a little more forward thinking than these were.
My only other precaution to give is when replacing do not over tighten the new plugs.
Good Day #oldnotdeadyet
Just Google “Ford 3 valve spark plug” and you’ll see you’re not alone.
What the heck were they thinking when they designed this head??
I think the Ford engineers should be required to replace all the spark plugs from those trucks. The design problem will never happen again.
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Allegedly, Preston Tucker told Alex Tremulis, the designer of the Tucker automobile, “I will make you attend the funeral of anyone killed in one of these cars”. The story may well be apocryphal, but supposedly this led to Tremulis taking extra care to provide good passenger protection in the event of a crash.
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