In May, Elon Musk made another of the bold projections that have become commonplace for the Tesla Motors Inc. chief executive when he said the electric car maker expected to deliver about 17,000 vehicles to buyers in the second quarter, its biggest ever.

In July, Tesla did something else that has become commonplace: It missed Mr. Musk’s target. Tesla said it delivered 14,370 vehicles, blaming the 15% shortfall partly on cars that were still on trucks and ships at the end of June.

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