The above chart shows the inflation-adjusted value of the S&P 500 index and the For-Hire Truck Tonnage Index (seasonally adjusted) that has been compiled since 1973 by the American Trucking Association. This latter is based on the amount of tonnage of all types carried by a wide array of trucking companies, both large and small. I see it as a proxy for the physical size of the economy, which is less than the total size of the economy since non-physical stuff (e.g., software, email, digital transmissions of movies, newspapers, magazines, and books) has become increasingly important.
The chart above zooms in on the same truck tonnage index over the past 7 years. Since the recovery got underway in mid-2009, the trucking index has increased at an annualized pace of 5.3%, which is substantially better than the 2.3% annualized pace of real GDP growth over the same period. I think the Truck Tonnage index provides solid support for the view that the U.S. economy is indeed growing, and probably doing somewhat better than many suspect.
As I noted before, the "overshoot" of stocks relative to the trucking index in 2000, and their subsequent "undershoot" in 2008, help us understand when the stock market is being influenced more by extremes of sentiment than by the economy's fundamentals. Today it looks like stocks are fairly valued given the underlying strength of the physical economy.