amount of work that can be done using an given quantity of gasoline == efficiency
The muscle cars turn the more efficient burning of gasoline into more horsepower per gallon
I will stop arguing after this post.
US cars are generally less efficient as they pack much less power per liter capacity.
For example, the iconic Dodge Viper gets 600bhp for a 8.4 liter engine (and something like 14mpg) while the Bugatti Veyron SS packs 1182bhp for 8 liter engine. If you want near the power of the Viper without turbos, just take the Ferrari 458 which packs 562hp with only 4.5L and 17.7mpg. Mean mpg doesn't mean much as it's not calculated based on the engine running at full power.
But again, it's not the same market. In Europe most people generally do not want a very big overtaxed engine with poor efficiency. Also our roads are not as wide and straight as in the US, so we focus more on reasonable power with good handling.