Tires: Tire size conversion, tire size conversion, designation change

Question
When recently buying new tires for my boat trailer I found that the tire industry is doing away with the older "alpha" designations (Where the letter indicates air chamber size) and replacing with metric designations. I found a crossover table which, for example, says that the old F78 -14ST = the new ST205/75D-14.  My question is why, if the change is simply a change in the way of expressing the size, did the aspect ratio change from 78 to 75? This would imply that there was a more fundamental change in the tire structure rather than just a designation change?


Answer
Dex,

You hit the nail on the head.

The Alpha designations were a by product of the mid 1970's, when 78 series tires were common.  With so many molds being obsoleted, some decided that using these molds for trailer tires would be very cost effective.

But these molds are getting worn out, so it is time to move on.

BTW the new desination for ST (Special Trailer tires) is based on a European system.