A famous ad from Humble Oil – which would later become ExxonMobil – urged motorists to “put a tiger in your tank.” More than half a century after it first appeared, that phrase is still part of the popular culture.

But, these days, there’s a little less tiger in every tank when it comes to the gasoline sold in the United States, and consumers have Washington policymakers to blame for that.

The reason is ethanol, which the federal government insists be blended into every gallon of motor fuel refined and sold in the United States. Mandates in recent energy legislation have ordered increasing amounts of ethanol in the nation’s fuel mix. The effect has been to lower the energy content of fuel.

According to the Energy Information Administration in its latest Monthly Energy Review, as the percentage of ethanol and other oxygenates added to gasoline has increased from 2 percent to 10 percent over the past two decades, the energy content of gasoline has fallen about 3 percent.

That’s because ethanol has just two-thirds the energy contained in a gallon of pure gasoline.

Because of congressional mandates and their implementing regulations, most U.S. motorists today are driving with gasoline with 10 percent ethanol content – the so-called E10 you see on stickers at the pump. That’s the highest percentage of ethanol in the fuel supply at any time since Henry Ford started mass-producing the Model T.

The math is simple: More ethanol = lower fuel economy = more fill-ups.

Looked at another way, it means that on top of the 40-to-60 cents per gallon paid in taxes, government is adding another cost that amounts to an extra $3 for every $100 spent by reducing the distance consumers get out of every tank of gasoline they buy.

Trade groups representing the renewable fuels industry are petitioning the federal government to extend its mandate. They want Washington to force refiners like ExxonMobil to blend 15 percent ethanol – or E15 – into the motor-fuel supply.

Obviously, such a proposal would lower fuel economy even further. It could also put many automobile engines at risk.

Popular Mechanics notes that ethanol “is fairly corrosive to rubber and certain metals, so it can cause damage to vital components. … Damage is also possible in fuel lines, injectors, seals, gaskets, and valve seats as well as carburetors on older engines.”

The dose makes the poison, as the old saying goes. Currently, blending up to 10 percent ethanol is deemed relatively safe for modern car engines. But according to AAA, a number of automobile manufacturers have suggested that their warranties won’t cover E15 for many vehicles in use today (not to mention marine and small engines, which are not guaranteed to operate safely on ethanol blends higher than E10).

As a AAA spokesman wrote last year, “The vast majority of cars on the roads today are not designed to run on gasoline containing more than 10 percent ethanol. While ethanol has the potential to support the economy and reduce the reliance on fossil fuels, it is irresponsible to mandate more ethanol than cars can safely use.”

For more than a decade, ethanol has taken the tiger out of too many tanks.  The residue it has left behind has reduced miles per gallon, the efficiency of engines, and helped empty the wallets of consumers as well as taxpayers.