The Fuel Gauge Matters More Than the Speedometer
Imagine a race car charging down the straightaway. The driver still has a foot on the gas. The engine is near the redline, and the crowd is cheering. But the fuel light is on. There may be enough in the tank to finish another lap at full speed. There may even be enough to win. What the driver cannot afford to ignore is that the margin for error has narrowed.
That is a useful way to think about stock valuations after a long market advance. Strong momentum tells us how fast the car is moving. Valuation tells us something about the fuel available to justify the price. Neither tells us exactly where the finish line is.
Our first chart compares the earnings yield of the S&P 500, using Robert Shiller’s CAPE measure, with the nominal yield on a 10-year U.S. Treasury.
Figure 1 Shiller CAPE earnings yield minus the nominal 10-year Treasury yield, with S&P 500 price.
What does CAPE actually measure?
CAPE stands for cyclically adjusted price-to-earnings ratio. An ordinary P/E ratio compares a stock’s price with a single year of earnings. CAPE instead compares the S&P 500’s price with an average of the previous ten years of earnings, adjusted for inflation. The longer window helps keep an unusually strong or weak year from dominating the calculation.
An earnings yield simply turns that ratio upside down. A CAPE of 40, for example, produces a CAPE earnings yield of 2.5%: 100 divided by 40. Think of it as a way to express the market’s normalized earnings relative to the price investors pay. It is not a dividend yield, cash paid to shareholders, or a promised annual return.
What the history shows
The chart contains 366 monthly observations from January 1996 through June 2026, the same window as the companion cash-flow chart. The shaded lower region begins below the 10th percentile, −2.84 points; the upper region begins above the 90th percentile, +2.38 points. These cutoffs represent “extreme readings” in the data set. The September 2026 spread was approximately −2.48 points. That is well below this window’s historical average.
The marked episodes show why this comparison deserves attention. Near the dot-com bubble, the spread fell to −4.38 points in January 2000. Before the global financial crisis, it reached a local low of −1.46 points in June 2007. That 2007 low did not enter the bottom decile; a market can become vulnerable without this indicator flashing its most extreme reading. This is not a market-timing signal, but rather a gauge of whether bonds or other assets might appear more attractive to investors and siphon off fuel from the stock market rally.
The other side of the chart matters too. As stocks fell and yields changed, the spread rose to +4.69 points in March 2009, near the financial-crisis market bottom, and peaked locally at +3.20 points in April 2020, one month after the COVID market low. Wide positive spreads can appear when fear is high and future equity returns may look more attractive, but uncertainty is also substantial. These are monthly spread turning points near market events, not proof that the spread caused or precisely predicted the market turns.
A second check: corporate cash flow versus corporate bonds
The companion chart (below) asks a related question from a different angle. It divides a broad measure of U.S. corporate cash flow, including net dividends, by the market value of domestic corporate equities. It then compares that aggregate cash-flow yield with the yield on a 10-year high-quality corporate bond. This cash-flow measure is not the same as Shiller’s CAPE earnings yield, and it is not cash an individual stockholder receives.
Figure 2 Corporate cash-flow yield and the 10-year high-quality corporate bond yield, with their spread.
Think of this comparison as a way to determine how much cash an investor could expect to receive for the investment. A stock investor receives (via ownership) a pro-rata portion of the net cash flow generated by the company, while a bond investor receives interest payments from the borrower. A stock, due to its highly variable price movement, should require a much higher income yield than a bond investment.
Across much of the chart’s 30-year history, the corporate cash-flow yield stood roughly six percentage points above the 10-year corporate bond yield. The gap varied widely, but it often gave stocks a substantial apparent yield advantage for the additional uncertainty of ownership. At the chart’s second-quarter 2026 endpoint, the cash-flow yield is about 6.08%, versus 5.72% for the corporate bond measure: a gap of just 0.37%. The spread came very close to zero around 2000, so today’s reading is nearing the narrowest in the chart’s full 30-year history.
Corporate cash-flow yield is an aggregate accounting measure, while bond yield is a quoted rate; realized returns on either investment depend on what happens afterward. But the narrowing gap tells us that, on this measure, the potential reward for taking equity risk looks much less generous relative to high-quality corporate bonds than it has at many points in the past.
Some investors may reasonably look at that narrower gap and decide that high-quality bonds deserve a larger place alongside stocks. Bonds offer contractual interest and a higher claim on a company’s assets; they still carry interest-rate and credit risk. Stocks offer participation in future growth, but their price can fall sharply even when the underlying business remains sound. The two yields measure different things and should not be treated as a direct forecast of which asset will win.
The racing analogy has a practical ending. A fuel warning does not require a driver to slam on the brakes, and it does not prove the race will end on the next turn. It calls for checking the gauges. For investors, that means reviewing whether equity exposure has grown beyond its intended weight, how much volatility the plan can absorb, what cash will be needed in the next few years, and whether bonds now offer enough income for the risks they carry.
At either extreme of the distribution, the right response is greater attention, not false certainty. The rally may have another lap in it. The portfolio should be prepared whether it does or it does not.
Data sources: Shiller CAPE and S&P price; FRED DGS10, CNCF, DIVIDEND, BOGZ1LM883164105Q, and HQMCB10YRP. SPX price excludes dividends; Shiller’s 2026 inputs may include estimates.
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