[fsstnotch]

My guess... 10mm is less than 1/2 inch. The specs just aren't accurate.

All the sites i looked at said they were almost identical. Comparitively equal on a 10" wheel they measured

325/50-15
11" tread width
13" section width
weighs 36lbs
overall diameter equal to 28"x13.50"-15

315/60-15
11" tread width
13.4" section width (due to larger sidewall)
weighs 40lbs
overall diameter equal to 30"x13.50"-15

SO... depending on what you plan to use the tire for, a taller/heavier tire could hurt quite a bit.