My truck is an 05 f-150 fx4 supercrew with 110***miles, 35x12.5x17 the speedometer isnt calibrated yet. The truck down shifts hard into second gear and when I'm traveling at speeds between 48 and 55 it feels like the tranny is slipping...any ideas?

 

More likely a low grade misfire. At 110k, it needs new spark plugs if they have never been changed.