275/60R15 Nitto NT555R, the factory specs are: 7.5"-9.0", but the tire is actually measured on 8" rim, giving overall width of 11.10".

I take that to mean:
- I could run on a rim in that 7.5-9" range
- the specs as far as sidewall to sidewall distance (so that 11.1" measurement) are achived when mounted on 8" rim

As others have pointed out, a wider rim will put more of the thread on the pavement, so usually better for traction.