We’re sorry for the confusion.
This lift is designed to fit a max outer tire width of 39.5 inches, but the usable width after correct assembly is about 26 inches, which is normal for this model.
Your John Deere S140 48" deck has a rear wheel width within 26", so it is compatible.
The 39.5" refers to the theoretical maximum adjustable range, not the actual working width.
By vevor