Garmin's best-selling devices are starting to look quaintly obsolete.

What: Shares of inveterate navigation tool maker Garmin (NASDAQ:GRMN) were down 14% at 2:30 p.m. Thursday, setting fresh multi-year lows during the trading session. Due to report third-quarter results in two weeks, the company jumped the gun with a preliminary update that fell short of analyst expectations.

So what: Garmin now expects to deliver adjusted earnings of roughly $0.51 per share on $680 million in total sales. Your average Wall Street forecast currently calls for earnings near $0.64 per share on something like $705 million in sales. In short, Garmin is falling far short of earlier expectations.

Now what: Management took pains to underscore that its fitness segment is showing fantastic growth. That unit is on track for 23% year-over-year sales growth in the third quarter, even as the company as a whole heads for 3.7% revenue shrinkage.

The fitness success is overshadowed by rapidly falling automotive navigation sales. Outdoor and aviation tools are also losing steam faster than expected.

Moreover, Garmin is fighting the same currency headwinds as everyone else, but from an unusually disadvantaged position. Based in Switzerland but doing most of its actual business in the Americas, Garmin feels more conversion pain than many other global businesses from the strong dollar as it converts it sales to dollars instead of Swiss francs.

But wait -- there's more. Even the fitness segment isn't quite living up to Garmin's own expectations. Management lowered its full-year growth expectations for that division from 25% to 15%, driven by difficult "pricing dynamics" in a difficult market.

Although we don't believe in timing the market or panicking over market movements, we do like to keep an eye on big changes -- just in case they're material to our investing thesis.

Meanwhile fitness-tracker rival Fitbit (NYSE:FIT) recently more than tripled its quarterly sales year over year, proving that there's room for high-octane growth in that specific market. If Garmin isn't taking advantage of that opportunity, chances are good that Fitbit and other mobile gadgetry gurus are stealing Garmin's potential sales from right under the company's nose.

Naturally, Fitbit shares rose more than 3% on Garmin's disappointing update, far ahead of Thursday's generally positive market tenor.

Garmin shares have now fallen 40% lower in 2015, and some 8% of the stock's float is still sold short as many bearish investors expect the decline to continue even further.

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