im not sure when to change the spark plug, but i think mine needs a change (27000 miles)
what about denso iridium? better performance? mile/gallon?

 

I am happy with my Denso Iridiums, but I think the heat range is more important than the type of plug. I feel that the stock plugs are too hot, which causes the ECU to pull timing and add fuel in the summer. I used the IK24's, and my car seems to run much more consistently in the heat.