How did I come up with 82?
It's just the aspect ratio that was used when this type of sizing was in place. Sort of a Tire guy's trivia question. EVERTHING was 82 aspect ratio - and bias ply, too!
When radials came along, the first sizes conformed to that sizing standard, untill it was discovered that you could use lower aspect ratios with radials and can some very real benefits. We've been headed downward ever since.
Hope this helps.