Picture a truck tire that has been driven flat. The body of it is still strong; only the grooved face that meets the road has worn smooth. So that face is stripped off and a new layer of rubber is bonded on, and the tire rolls out of the workshop looking new. That is the picture the word keeps returning to: an old thing with a fresh surface. Most of what retread does in English is that picture applied to something else — a story given a new title, a worker given new skills. One branch is separate: it is built on tread meaning to walk, and it means going over the same ground a second time.
From the word’s history
Wiktionary gives two entries. One is re- plus tread meaning the grooves carved into the face of a tire; the other is re- plus the verb tread, meaning to walk.
Wiktionary 把這個字分成兩個字源。一個是 re- 加上名詞 tread（輪胎表面刻出來的溝紋），輪胎和它的各種比喻都從這裡來。另一個是 re- 加上動詞 tread（踩、走），「再走一次」那一支從這裡來。兩支同拼法，連過去式都不一樣。
Wiktionary, "retread" (etymology 1)
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