Ford (NYSE: F) released its first-quarter earnings this morning, and once again the results were impressive: Net income was $2.1 billion, or $0.50 a share, a $3.5 billion improvement over last year and solidly ahead of the $0.31 a share expected by analysts.

This is Ford's fourth quarterly profit in a row and its highest quarterly operating profit in six years, further evidence -- as if any were really needed -- that the wrenching restructuring imposed by CEO Alan Mulally has turned the ancient auto giant into a solid modern business.

But now that we've seen several quarters' worth of upside surprises, growth, and improvements, one has to ask: How much more does the company have up its sleeve?

The key to Ford's rebound
While the global economic recovery (such as it is) has been part of Ford's recovery story, the biggest source of the company's success is simply that its products are more competitive than they had been.

On a conference call for media and analysts this morning, talking about the company's gains in different segments and markets around the world, Ford executives had one refrain: Gains were because of "higher volume and favorable net pricing." Translation: We sold more car and trucks, and because they were seen as top-notch products, we were able to get more money for them.

While the company still has a significant amount of debt to pay down, much of the pain of restructuring is behind it, with headcounts in particular reaching a "stable" point, Chief Financial Officer Lewis Booth said this morning. And the fruits of the globalization of Ford's product line -- improved products and improved economies of scale -- are largely in place. The new globalized product platforms are either out or due shortly, with the one major piece -- the upcoming Ford Focus and its derivatives -- set to be released in the U.S. and Europe early next year.

All of this is good, and the results are in the numbers. Ford's auto business was profitable in every region of the world, and management expects "solid profits" for the year. The signs suggest that Mulally and his team have succeeded in turning Ford around, and with good management and continued investment in new products, the company should be sustainable over the long haul.

That's a very impressive achievement, given where Ford was a few years ago.

But here's the big question that should be on Ford shareholders' minds: Where does the company go from here? Or put another way: What can we expect for the stock?

Where will Ford find growth?
At some point, Ford stops being a recovery story and starts being just a business, and that business is a sprawling global company with high fixed costs in a cyclical, cutthroat industry. Incremental market share gains in the U.S. and Europe will be nice to see, and they'll add to the bottom line as consumer spending in those markets continues to improve, but they're not going to drive $3 billion-plus improvements in net income in future quarters. If anything, competitive pressures and rising commodity prices are likely to mute future results somewhat.

And while Ford is likely to see more dramatic growth overseas, particularly in the rapidly expanding Chinese and Indian markets, it won't be the only one looking for such growth. Toyota (NYSE: TM) and Honda (NYSE: HMC) can be expected to raise their games in China in coming years as they, and a host of emerging Chinese firms, aim for market leaders Volkswagen and General Motors right along with Ford.

India's market is at a somewhat earlier stage of development, meaning that Ford's recent push there could yield bigger results over time, but all of the key players in India will have to deal with the low pricing expectations set by Tata Motors' (NYSE: TTM) new $2,500 Nano.

Long story short, Ford should be able to grow sales -- and profits -- over time, but the "low-hanging fruit" of the turnaround is largely gathered, and the implications for shareholders are obvious.

Expectations for shareholders
Fool Rich Smith noted recently that Ford's share price is roughly 0.4 times sales, lower than Toyota's at 0.66, Honda's at 0.72, and even Tata's at around 0.6. I'd expect Ford's ratio to come more into line with the Japanese giants' as Ford's sales gains solidify and its debt continues to come down, and as Ford starts to be seen more as a competitive global automaker and less as an embattled Detroit company.

There's still some upside to the stock, in other words; at some point, perhaps after the debt is reduced, there should be dividends as well. But just as with the company, the stock's big gains seem largely behind us. Next quarter may well be as profitable as this one, but the comparison, for the first time in a long time, will be to another profitable quarter last year. The news from here on out is likely to be less exciting.

So here it is: I think Ford stock is still worth holding. I have no plans to sell mine. But I also think that the time has come for shareholders -- particularly those who bought in on the turnaround story -- to mentally reassign the stock to the big-and-boring corner of their portfolio.

What do you think? Is Ford's turnaround largely complete or is there still work to be done? Scroll down to leave a comment and let me know.

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Fool contributor John Rosevear owns Ford preferred shares. Ford is a Motley Fool Stock Advisor recommendation. Try any of our Foolish newsletters today, free for 30 days. The Motley Fool has a disclosure policy.