Trek 56 or Specialized 54? Find what size you wear in any other road, gravel, or endurance frame, based on the geometry numbers (reach + stack) rather than the label on the down tube.
How to read this.The distance score is a weighted Euclidean distance on (reach, stack), with stack weighted 1.5× because stack differences are easier to compensate for with spacers and stem flips. A score of 0 means the geometry is identical; ≤ 5mm is "on target"; 5-15mm is close enough to try; > 15mm usually means re-fitting rather than swapping labels. Reach differences are the most important — a 10mm reach delta usually requires a different size, not a different stem.