Predictions welcome. I never actually raced a 400 at a meet or in practice so I'm just curious of what I was capable of. Didn't race 800s open either, only in 4x8's where my best split was about a 2:07 or 2:08. Other PRs 4:40, 10:26, 16:46. My 100/200 speed wasn't very good but not terrible either. Not sure where I'd top out there.
57
When my PRs were similar (2:06/4:41) my 400 PR was 53.5, that said I was always more of a speed based runner.
I'd say 56-57 is a good guess
I was a 2:05/4:39 guy and split 55 in a couple 4x4 relay legs. I agree with the other posters. ~56 in an open 400
in high school, I ran 2:12/4:55y/10:39y and coach thought I couldn't break 60, so I ran 58 in a time trial after the season. All this on cinders 440y. However, in college, I ran 2:05.8/4:39/9:33 3k/16:20 5k and in my last 800m with only pool running behind me I was out quick, saw 49 on the clock with maybe 50m to go to the bell and decided to slow way down so as not to hit 400m too fast (probably on pace for about 56), thinking I'd rig if I didn't slow way down (this was not a good strategy--if you're in, you're in and commit all the way).
In other words, do some speedwork for a few months and you may surprise yourself.
52.9-54.1