Tesla Inc., Elon Musk’s 17-year-old upstart carmaker, took a giant step toward blue-chip respectability, getting named to one of the world’s most famous stock indexes in an action that will greatly broaden its investor base.

Analysts at Credit Suisse and RBC Capital Markets are among those predicting Tesla will post lower revenue for the third quarter than a year ago. If there is indeed a decline, it will be a first for the company since 2012 — the year the Model S started production.