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.

What: Shares of truck builder Paccar (Nasdaq: PCAR) slipped into neutral and coasted down a very steep hill Tuesday, losing nearly 11% when all was said and done.

So what: As you can probably guess, Paccar reported earnings this morning. Revenues rose 61% year over year. Profits more than doubled in comparison with last year's Q2, hitting $0.65. Problem was, they were supposed to hit $0.69 (in Wall Street's opinion.)

Now what: Don't say I didn't tell you so -- because I did. Expectations were big for Paccar this year, as the company benefits from a trucking renaissance in America. Trucking companies such as Con-way (NYSE: CNW) and Heartland Express (Nasdaq: HTLD) are grabbing semis hand over fist. But the problem with Paccar is that it was priced for perfection in this nearly perfect seller's market.

Indeed, even after today's selloff, the stock costs nearly 23 times earnings -- a pretty premium to Street expectations of sub-20% earnings growth. The stock's a bit cheaper if valued on its free cash flow -- but still not as cheap as Navistar (NYSE: NAV), which generates more free cash than Paccar but sells for only a fraction of its rival's market cap.

My advice: If you want your portfolio to keep on trucking, ditch Paccar and hitch up to Navistar. It'll go far. 

Which is the better buy, Paccar or Navistar? Add both stocks to your Watchlist and find out.