At one store I operated, the premium tank was so small that it would hardly take the minium amount of fuel we could order. Often once it was full, we dumped the extra premium fuel into the mid grade tank. as a result it was certain the mid grade was higher than 89 octane most of the time.
When we remodeled the store, we went two large tanks with pumps that blended the premium and regular to make mid grade. This eliminated the problem of the small premiium tank.