Yes, fuel efficiency means how much work is done (e.g., a 20 mpg 2 ton vehicle is twice a fuel efficient as a 20 mpg 1 ton vehicle); fuel economy means how far does a vehicle go on a volume of fuel. The similarity between the two is often used by propagandists to confuse people. The EPA City/Hwy fuel economy ratings (and others in different countries) should be one's yardstick.

(Corrected: I wondered why I received negative scores)

The distinction between "economy" and "efficiency" is astute and relevant.

A Ford Model-T got 20 mpg. Economically, it does about as well as modern pickup truck. However, a modern pickup will haul much more (including its own weight) than a Model-T could ever hope to carry for the same fuel. Thus a modern truck is more efficient. The increases in efficiency mean that more of the fuel is converted to power and less to waste heat.

Had the increases in efficiency over the last decade been reinvested into cars weighing a ton or less, people would easily be averaging 40 mpg or more. For example, in college I had a 1983 Subaru Wagon -- it got 30 mpg (less in 4wd). It wasn't a race car by any means, but it did the job. As engines became more efficient however, car manufacturers used that efficiency to make cars larger, or increase horsepower because consumers like the feeling of acceleration when they stomp on the gas, or the smooth ride of a 4000 pound beast.

Manufacturers could have used that efficiency to make smaller motors, keep the power around 80 horses, and in so doing, create cars that directed the efficiency gains toward gas savings, i.e., "economized" on fuel usage. Indeed, in many parts of the world, regular ICE powered cars get 60 or 70 mpg right now in a perfect example of how efficiency can be translated into more horses OR less fuel usage.

Anyway, I've suspected that cars have become more efficient because really big vehicles do a whole lot better than the really big '76 Impala I had in HS (10 mpg), and it has frustrated me that it's virtually impossible to find anything better than 35 mpg when even in the 80s, such cars were all around.

Surely a 2 tonne vehicle which uses 1 gallon of fuel to move 50 miles is no more efficient than a 1 tonne vehicle which uses 1 gallon of fuel to move 50 miles. Both are using the same amount of fuel to do the same job.

A perfectly valid question, yet currently rated -1, and nobody answered his question. Get a life people.

Your reasoning is correct IF THE VEHICLE IS TRAVELING AT CONSTANT SPEED. (Sorry for the caps, but that point needs emphasizing.) The problem comes in when you have to accelerate. Since F=ma, it takes twice the force (and therefore twice the work since W=Fd) to bring the 2 tonne car up to your desired speed as compared to the 1 tonne car. Every time you accelerate you're using at least twice as much fuel, all other things being equal (e.g., engine, aerodynamics etc...) That is one reason why SUV's are less fuel efficient than passenger cars.

However, if you're smart, you probably realize this is not the whole story because if it was, SUV's should get the same highway mileage as a toyota corolla. However, they don't...because a) people don't travel at a constant speed on the highway and I believe the new rules on quoting mileage require the companies to quote mileage under "real life" driving conditions. b) SUV's have absolutely useless aerodynamics and aerodynamics is the major factor contributing to fuel consumption on the highway.

I hope you're not a troll, but even if you are, there is my honest answer to your question.