I know I am new here but in my experience, Bosch Iridium Fusion +4 plugs are the best for higher MPGs. The spark gaps are open to the entire combustion chamber giving the best, most complete ignition of the fuel air combination. I installed them in my 2000 Corolla CE in 2009 when I purchased it at 52,000 miles and saw an immediate increase in performance and efficiency. The engine also ran smoother. I started feeling a decrease in performance and efficiency right about 90,000 miles and replaced them with the platinum version. They worked almost as good so I swear by the iridiums.
My new 2005 LE will be getting a set next week, as soon as they get shipped to me.
One thing to note, do NOT skimp because they are more expensive. The fuel savings way more than pay for them in a couple of months and mine lasted almost 4 years. I think $6 per plug is worth the price for what you get out of them.
I purchased a set immediately for my new to me 05 Corolla LE and I am seeing a very respectable 34 MPG average. This is my third set of the Bosch Iridium Fusion +4 plugs, 2 sets in my 2000 and this set. On a whim I pulled these last month on the one year anniversary and was amazed to find them completely clean. Yes, they look a little used being a little discolored, but there was no ash or carbon deposits, nor any evidence of overheating or detonation. Maybe I have just had good luck with them.