If it’s an 8 lug nut 3/4 ton, 12 mpg would be about the best you could expect if back to original. If it’s a 5 lug, 14+ might be possible. I really can’t understand why anyone would waste the time and money to install high performance parts on such a work dedicated truck. It’s kinda like me spending $500 on a pair of track shoes and wearing them to push a car around the shop.