Is iridium plugs designed for better performance/durability than fuel economy? A friend's verna was showing fuel economy drop (Already FE is pathetic in petrol verna, so a drop post iridium usage is again ).
Iridium plugs will definitely give better mileage as the combustion is more complete. Maybe your friend's driving style might have changed slightly because the throttle response is better. Or it can be a complete different factor affecting it.