As 2015 begins, Tesla (NASDAQ:TSLA) investors can look back in retrospect at another great year of returns. Shares are up close to 50%, easily trumping the S&P 500's 13% gain. But while the stock's gain is great for Tesla shareholders, the rise is bringing the stock's incredibly bullish valuation into the spotlight. This is why investors will be looking to Tesla's vehicle sales in 2015 to validate the market's optimism about its future. The metric's trajectory in 2015 will help answer the unavoidable question: Can Tesla continue to serve up the sort of growth required to justify its pricey $29 billion valuation?

Can Tesla keep up its growth?
Tesla is undoubtedly growing extraordinarily fast. Just take a look at the electric-car maker's vehicle sales growth since 2012:

Asterisk indicates an estimate. The 2014 estimate is based on three quarters of available data on vehicle sales combined with fourth-quarter guidance. Data for chart is from SEC filings.

Driven by the introduction of the Model S in the second half of 2012, Tesla's vehicle sales have been rising steeply. And, according to Tesla, this growth trajectory won't be slowing any time soon. A few recent remarks from Tesla management provide insight about what the company believes its vehicle sales could look like in the coming years.

"[W]e are confident of a 50% increase in both net orders and deliveries for Model S alone in 2015," Tesla said in its third-quarter letter to shareholders. And this estimate doesn't even factor in the potential for Tesla's Model X deliveries in 2015. The all-electric SUV is supposed to hit the market in the third quarter of 2015.

"It would be some number greater than 50% if you include X," Tesla CEO Elon Musk said during the company's third-quarter earnings call. "I am just reluctant to -- I can't communicate a greater level of certainty than I actually know. That's why I'm reluctant to give a number."

But given that it wasn't until Tesla surpassed 20,000 Model X orders that the company started shifting estimated delivery dates for new orders to "early 2016," 10,000 to 20,000 Model X deliveries in 2015 seems like a reasonable estimate.

Model S. Today, the Model S accounts for 100% of Tesla's vehicle sales. Image source: Tesla Motors.

Going beyond 2015, Musk said during Tesla's earnings call that the company is confident that it will be able to grow orders and deliveries by 50% "in 2016 and in subsequent years, too."

Point is, Tesla management has set the stakes high -- and the market will be waiting for Tesla to deliver.

What vehicle sales should we expect from Tesla in 2015?
To live up to the hype, Tesla will need to deliver around 70,000 this year, or around 50,000 Model S and 20,000 Model X. This would mean Tesla's year-over-year vehicle sales growth would actually accelerate from a rate of around 45% in 2014 to growth in excess of 100%.

But is this sort of growth realistic?

If Tesla's short history is any indication of the future, a forecast for 50,000 Model S deliveries in 2015 will likely be close to reality. So far, Tesla has done a solid job at living up to its guidance. Going into 2013, Tesla predicted it would deliver 20,000 vehicles. By year-end, sales were close to 22,500. For 2014, Tesla initially guided for 35,000 vehicle deliveries. But when it reported third-quarter results, it scaled back its annual guidance to 33,000, citing difficulties in production that put the company 2,000 cars behind headed into Q4. Assuming Tesla did deliver 33,000 vehicles in 2014 (we'll find out when the company reports Q4 results), Tesla 2013 and 2014 deliveries combined are only 500 vehicles short of its initial guidance for the two years.

But the Model X deliveries for 2015 is the big wildcard. Tesla hasn't yet provided any official guidance for Model X deliveries. And while Tesla did say the ramp-up in production for the X would be faster than the ramp-up in production for the S, 20,000 deliveries in six months would definitely be an impressive feat.

Model S (left) and prototype of Model X. Source: Tesla Motors.

Given that Tesla has more than enough demand, investors should look for comments from management in Tesla's upcoming fourth-quarter letter to shareholders on its predicted production ramp-up for 2015. Limited by supply -- and not demand -- Tesla's production will ultimately determine vehicle sales.

Beyond 2015, Tesla investors expect even more growth, driven by Tesla's upcoming lower-cost Model 3 and increased lithium-ion supply from its under-construction Gigafactory. But, for now, Tesla will need to show the world it can deliver on its short-term growth goals.