"When real (inflation-adjusted) wages are stagnant and the price of gasoline is high, as was the case in the late 1970s and the recessionary early 1980s, the ratio is low. If income is stagnant and the cost of gasoline is high, then people have less money to spend on other items and the economy is also stagnant--exactly what occurred after the 1979 Iran Crisis pushed gasoline prices up. (Sound familiar?)
"When gasoline is relatively cheap and incomes are rising, then the ratio is high. Thus when oil prices hit bottom and incomes were rising in the late 1990s, then the ratio was peaking."