For 2015 Ford says 17" wheels have a 3195 lb rating.
18"-20" wheels at 3590 lbs.
A 3750 lb rated tire is a good choice for your OEM 3590 lb rated wheels. Just don't exceed a tire or wheel load rating.
A 3900 lb payload sticker on a F350 SRW ??
This is a gvwr based payload. In many cases if its all placed in the bed it will overload the lessor of a tire/wheel/rear spring pack.
Run some numbers at ya'.
Your truck rear axle may weigh in the 3400-3500 lb range before loading. Now add a 3900 lb load in the bed = 7400 lbs on the rear axle. Just one reason payloads are ignored for loads carried by the rear axle. Another is its not a legal weight issue with dot.
Your scale ticket before loading will give you how much capacity you will have to work with. I like a good 15% reserve capacity above max loads for my tires and wheels.
