[thstone]

The problem is that there are far too many other variables that you can't keep constant in order to say that the spark plug change was the cause for such a small change in temp as shown on the gauge cluster. Even if you had Durametric data, there are still far too many variables in play.

That is why people are saying "placebo" - they don't mean that you're not seeing a difference on the temp gauge, instead they are saying that it is highly likely NOT the spark plugs and much more likely one or more other variables that can affect the reported coolant temp.