Update: I recently flushed my trans and replaced the fluid with SP III and now my mileage is right where it should be. I'm now getting between 15 and 17 and the truck drives smoother. BTW, SP III fluid is used by Hyundai and Kia too. The nearest Mitsubishi dealer to me was like 150 miles away but we had a Kia dealer in town so I just got it there.