2004 Dodge Ram Low Fuel Mileage

I always did get low fuel mileage, about 14 mpg. But recently it has dropped to about 12.9. It's been 30,000 miles since last plug change. Should I change to these Iridium or Platinum plugs or should I stick with the Oem plugs? Also, is synthetic oil worth the extra cost? Should my plug wires be changed. They are still the original. Any advice or other suggestions to improve fuel mileage would be appreciated.