I have a 21 ft. lowes pontoon with a 60hp evinrude. I put new 10 inch tires on the trailer last year. Just the "normal" wide pontoon tires you see on most single axle pontoons. These tires have a D load rating. The problem I'm having is that both tires have worn down about the 1st inch (width) on the inside of the tread. Rest of tire looks new still. I have changed bearings and etc. on both sides. My question is, does anyone have any idea(s) on what may be causing this, and a way to fix said problem ? Any help would be thankful. Thank You.

Your axle is not big enough. Wearing on the inside edge is normally from axle bow. I have had many a trailer do that when i overloaded it and than the inside tires would wear out. And go bald. Put a bigger axle under it. I would look to put 15 inch tires on it with a 1750 or 1500 spring set up

Yep. bigger axle and tires..make sure its a drop axle to keep the pontoon boat at or near the same height.

Agree. Had to upsize the axle on my Sea Pro trailer years back that was doing the same thong.

Mine does the same thing, now I know why.