I'm 5'8" and ride a 54 cm hybrid commuter. But on a road bike (different geometry) I ride 50 or 52.

Can you put your Trek and Clunker side-by-side and compare important points? Seat, cranks, bars... do they match? Use the bottom bracket (axle of the cranks) as your reference point, since you can't move that. Does it look like you could move the seat and bars of the Trek to make it match the clunker?

You have a unique body geometry, and clearly the standard "correct" bike fit isn't working for you. If the clunker's set-up IS working for you, it might make sense to use the clunker as your template.