At least in my country, the amounts spent on gas monthly hasn't really been dropping, even as cars become more efficient. I'm not familiar with this area, but

At least in my country, the amounts spent on gas monthly hasn't really been dropping, even as cars become more efficient. I'm not familiar with this area, but I think that in the US retail gasoline prices (that is, including all taxes) have been historically flat. This means that efficiency gains fairly directly to lower operating costs, and hence result in greater demand: "In constant dollar terms, the price of gasoline in 2015 was only seven cents higher than in 1929." https://energy.gov/eere/vehicles/fact-915-march-7-2016-average-historical-annual-gasoline-pump-price-1929-2015 https://energy.gov/eere/vehicles/fact-915-march-7-2016-avera... I don't think that fuel taxes have increased much on a constant-dollar per-gallon basis. Considering inflation, the federal rate has in fact dropped significantly: "Since 1993, the US federal gasoline tax has been 18.4¢/gal (4.86¢/L)." https://en.wikipedia.org/wiki/Fuel_taxes_in_the_United_States https://en.wikipedia.org/wiki/Fuel_taxes_in_the_United_State... I don't know how the state rates have changed (they are of the same magnitude as the federal rate, ranging from 20-40¢/gal) but my guess would be that they too are historically fairly constant per gallon over the recent decades of efficiency improvements, resulting in an inflation adjusted decrease in the proportion of tax being paid.