I've noticed that super-singles sit a little more inwards. Seems to me that in strong cross-winds, that the tractor trailer is a little less stable (thinking physics).
If I were to put super-singles on our drive axle, the tag axle tires would stick out further unless there is an adaptor of some sort to even them up.
We have 365's on the steer and tag. The extra width of the 365 on the tag makes it even with the drive tires (315's). If you ever noticed a Greyhound bus and how the tag axle tires are more inward, they are probably running 315's all around.
Safe travels,
MM.