Owners manual says spark plugs are good for the life of the car. At 160K I am seeing about a ten per cent decrease in gas mileage. Any thoughts on how to tell if they should be replaced?

 

You should have had iridium spark plugs from the factory. However, I would have replaced the plugs 60K miles ago. See my response in the below link along with my picture of a new iridium verses one with 100K miles on it. Big difference in the gap (.80 for old vs .54 for new)

https://www.fordforum.com/forum/ford...k-plugs-30428/