Lucid Group (LCID 4.64%) is entering one of the most exciting years in its history. Following the recent launch of its Gravity SUV platform, analysts expect the company to more than double its sales this year. That's a big reason why shares are priced at a lofty 11 times sales.

If you're betting on this growth stock, make sure you also pay attention to the metric I talk about below. Long term, it could prove just as important as sales growth.

Lucid needs this number to improve

Creating and scaling an electric vehicle manufacturer is pricey. It takes billions of dollars to bring a new car model to market, and profits shouldn't be anticipated for years, if not decades. Companies like Tesla were able to achieve positive gross margins fairly early, but newcomers like Rivian have found it more difficult to turn a profit on each car sold. As the smallest of the three, Lucid remains well behind in this category.

RIVN Revenue (TTM) data by YCharts

While the market is paying close attention to Lucid's sales growth trajectory, gross margin can become a major factor at any time, especially if sales growth stalls unexpectedly. That's because gross margins are a reliable indicator of Lucid's cash flow needs. Tesla makes a profit on every car it sells, making it much easier to generate a net profit overall, and thus just as likely to generate free cash flow over time. Lucid, however, still loses money on every car it sells. That makes turning a companywide profit nearly impossible, typically leading to negative free-cash-flow generation.

For now, Lucid's rapid sales growth can keep the heat off of weak gross margin. But if you're a long-term investor in Lucid, sales growth and gross margin improvement are the two most important metrics to track.