What: Heavy-duty truck and engine manufacturer Navistar International Corp (NYSE:NAV) stock is getting run off the road today, down 18% as of this writing at 2 p.m. EST. This is more of the same ugliness for shareholders over the past few years, who have seen the company's stock lose more than 60% of its value since the beginning of 2014:

NAV Chart

NAV data by YCharts.

So what: There's absolutely no news out there to point at as an explanation for Navistar's sharp sell-off today, which is happening on exceedingly high share volume that's more than triple the three-month average. There's also not anything on the newswire about competitor Paccar (NASDAQ:PCAR) that could affect Navistar's stock price today. 

Chances are, what we are seeing is a combination of factors. This time of year, it's not uncommon to see individual investors and large funds and money managers offload stocks that are down for the year. Not only are there tax benefits from so-called tax-loss selling, but funds often sell off their losers to make their portfolios look better. 

For a stock like Navistar's, which is now down more than 71% this year, it's a prime end-of-year selling candidate. Sometimes momentum can take over, too, when a little bit of selling sends a stock lower, which leads to more selling. Before you know it, a beat-down stock gets even more beaten down. I think that's the situation here. 

Now what: With that said, I don't think Navistar is necessarily a candidate to buy on the dip. The company is still struggling, frankly, and competitor Paccar makes for a much better investment at these prices. 

Paccar's stock is also down this year -- about 28% -- but its business is performing quite well, especially compared to the struggling Navistar:

NAV Revenue (TTM) Chart

NAV Revenue (TTM) data by YCharts.

The heavy-truck business has rebounded strongly over the past 18 months, but Navistar has failed to benefit from that recovery, unlike Paccar, which has seen revenue and earnings increase by 23% and 61%, respectively, since the start of 2014. Over the same period, Navistar's revenue has declined 13%, and the company continues to report a net loss, though it has reduced its losses. 

Nonetheless, the heavy truck market is expected to remain relatively strong, with a solid U.S. economy and a healthy construction industry driving domestic results, and a steadily improving European economy lifting sales across the pond. 

Bottom line? Better to go with the higher-quality business in Paccar, versus the more beaten-down, still-struggling Navistar. 

This article represents the opinion of the writer, who may disagree with the “official” recommendation position of a Motley Fool premium advisory service. We’re motley! Questioning an investing thesis -- even one of our own -- helps us all think critically about investing and make decisions that help us become smarter, happier, and richer.