I don't think Zoro was ever stronger than Luffy but he was never really far off either. They've for the most part always been portrayed as relative to each other, and the databook or whatever had them both at Strength 6.
Here is how I view Luffy vs Zoro
Zoro was portrayed equal to Luffy in Whiskey Peak without Sanzen Sekkai
So Whiskey Peak Full Power Zoro high diffs Luffy
Alabasta only Zoro got power up not Luffy
Alabasta Zoro mid diffs Luffy
Skypea Only Zoro got power up not Luffy
Skypea Zoro low/mid diffs Luffy
Enies Lobby Extreme diff either way
Thriller Bark Zoro high diffs Luffy
Marine Ford Extreme diff either way
Post Time Skip Zoro mid diffs Luffy
chapter 1000 Zoro mid/high diffs Chapter 1000 Luffy
Chapter 1035 Zoro Mid/high diffs Chapter 1027 Luffy
Current Luffy extreme diffs Current Zoro.