Bald Tires...
In English when tires no longer have tread on them we say they are bald, just like a person with no hair.
In Spanish I have never heard calvo or pelado, the two words for bald that I am familiar with used in this manner. I have however, heard the word lisa (smooth) used in this manner.
Anyone have any other ideas about how to express bald tires?
5 Answers
llantas desgastadas? - from an automobile site.

- Llantas lisas
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Neumáticos lisos or llantas lisas (as already mentioned above).
For practice, you might consider reading about these ideas in context. Perhaps one of the following articles might be of some use:
Grammatically speaking, "desgastar" is "to wear" so to say "to wear out" we need an adverb or adjective to indicate "completely worn out". From the article in El Universo: Regularmente el excesivo desgaste de las llantas tiene que ver con problemas en el sistema de frenos y en la suspensión del automotor. (However, sometimes "desgastado" is incorrectly used to mean "completely worn out").
In this context it means the same as "bald tires" or "completely worn out tires." Also keep in mind that "bald tires" is idiomatic as tires don´t grow hair.
Another way to say "bald tires" is "llantas acabadas".
Note from the RAE:
desgastar. (De des- y gastar). 1. tr. Quitar o consumir poco a poco por el uso o el roce parte de algo. U. t. c. prnl.
Julian said: Also keep in mind that "bald tires" is idiomatic as tires don´t grow hair.
This is from the Oxford English dictionary. See 1.3. So it is not idiomatic at all.