For most of the past couple of years, electric car pioneer Tesla Motors (TSLA -12.34%) has been working toward an ambitious goal of producing 500,000 vehicles annually by 2020. Wall Street analysts have generally been skeptical of this target. This isn't too surprising, given that Tesla delivered just 50,580 last year.

Nevertheless, Tesla set its sights on even bolder targets back in May: It now aims to produce 500,000 cars annually beginning in 2018 and nearly 1 million a year by 2020.

Unfortunately, Tesla continues to routinely fall short of much easier targets. As a result, investors should probably expect it to miss these loftier new production goals by at least a year or two.

A long history of missing targets

Under Elon Musk, Tesla has managed to do the impossible: make electric vehicles exciting and hugely popular. However, it's been anything but a smooth ride.

Getting vehicles ready on time has been a constant challenge for Tesla. Image source: Tesla Motors.

First, both of Tesla's current models -- the Model S and Model X -- arrived much later than planned. The Model X was the worst offender, due to its over-engineered design. The first Model X was supposed to be delivered in 2013; instead, the first hand-off occurred two years later, in late 2015.

Second, while Tesla has increased its output every year, it has frequently missed its production growth targets. For example, in March 2015, Musk predicted that Tesla would deliver about 55,000 vehicles that year. It ultimately fell short of that number by more than 4,000 units.

Even when looking at short time horizons, Tesla has had trouble meeting its goals. In February 2016 -- almost halfway through the first quarter -- Tesla predicted that it would deliver 16,000 vehicles in Q1. It only delivered 14,820, blaming the shortfall on supplier unreliability and its own "hubris" in designing the Model X with too much technology.

Fixes on the way?

Tesla has stated that it is learning from its mistakes so that it can avoid repeating them with the upcoming Model 3. The company says it is focusing on making the Model 3 simpler to build in order to support a much higher planned production rate.

Numerous Wall Street analysts who have toured Tesla's factory in California claim the automaker has made dramatic progress in expanding production capacity in the past year or so. After it racked up hundreds of thousands of Model 3 reservations in just two weeks, staying on track has become more important than ever.

Tesla has received hundreds of thousands of Model 3 reservations. Image source: Tesla Motors.

On the other hand, as of early May, Tesla still hadn't finalized the Model 3 design. The company was aiming to finish that process by late June or early July. This means that Tesla is aiming to start production within about a year of freezing the design.

That isn't completely unheard of in the auto industry, but a two-year gap between freezing the design and starting volume production is far more typical. Given that mass automakers have far more expertise in high-volume production, it's pure hubris (again) for Tesla to expect to bring the Model 3 to market on this accelerated schedule. Thus, it's quite unlikely that the first Model 3 will be ready for delivery by the end of 2017.

Furthermore, once production starts, Tesla will be relying on suppliers to deliver components on time -- without sacrificing quality. This has been a recurring problem for its suppliers. At first, they simply weren't ready for Tesla's orders because they didn't really expect the company to succeed. Today, that's no longer an issue, but suppliers are probably just as skeptical as the analyst community regarding Tesla's plan to build 500,000 cars in 2018. If suppliers low-ball their own production in consequence, it could become a self-fulfilling prophecy, preventing Tesla from meeting its deliveries target.

One more missed goal

Just this past weekend, Tesla announced it had missed yet another goal. In May, it stated that it would build about 20,000 vehicles in Q2 and deliver 17,000 of them, with the rest remaining in transit at the end of the quarter.

Instead, it produced 18,345 vehicles and delivered just 14,370. Furthermore, Tesla now expects to deliver about 50,000 vehicles in the second half of 2016, which means it would fall short of its full-year target to deliver 80,000 to 90,000 vehicles (albeit not by very much).

When a company can't even accurately estimate its production a few months in advance, its foolish to give full credibility to its production forecast for 2018. That's no more than a wild (and optimistic) guess.

Clearly, Tesla will try to start building the Model 3 as soon as possible, and it will try to ramp up production as quickly as possible. But even if it addresses all of the issues that caused delays in the past, it will undoubtedly run into other unforeseen problems. Just reaching its previous goal of building 500,000 vehicles annually by 2020 may be the best that investors can reasonably hope for.