The runner I'm thinking of has not broken 51.
My 'calculator' is no calculator. These are conversions based on observations of real world performances.
This is not telling you what are 'equivalent' times, that is nonsense. If someone ran a 400 time 'equivalent' to their mile time their 800m would be much better than either.
This is telling you that looking at the 400 to 800 in isolation from the 800 to 1500 is insanity. The runner has run fast in the 400m and the mile.
I will explain again for you in plainer English.
Of course, 48.5 for a 1:48 runner is fine. I understand that, clearly from my post. That is the kind of conversion seen by Amos and Rudisha, do the math yourself, and that is perfectly fine for a speedy or pure 800m type.
Then you look at his 800m to his mile, and his conversion is better than Coe or Symmonds. I.e. he has more relative endurance than them. He can hold his 800m speed through to the mile better than Coe or Symmonds could.
That shows great aerobic strength.
It is not logical to have a runner that shows relatively poor endurance from 400-800m, then somehow have great endurance from 800 to the mile.
There are only two explanations; either he sucks at the 800m or he is capable of much faster.