There's a popular comparison going around the internet that conveniently links the gas mileage of a car made nearly 30 years ago with one made today.

That comparison involves the 1980s Honda CR-X HF, and the modern 2011 Honda CR-Z. At 41 mpg city and 50 mpg highway under current EPA ratings, the old car is actually 10 mpg and 13 mpg better than the CR-Z, respectively.

And Massachusetts Institute of Technology (MIT) has confirmed the fairly obvious explanation as to why the economy of average vehicles has failed to rise significantly, despite modern technology advantages: Weight.

Performance, too. As buyers ask for more of everything - more size, more space, more power, more comforts and conveniences - the modern car has grown massively compared to its old counterparts.

A new Honda CR-Z weighs in at over 2,500 lbs - light by modern standards, but still a massive 760 lbs more than that mid-eighties CR-X. At an increase of 35 percent, that's actually more than the 26 percent average weight increase calculated by MIT economist, Christopher Knittel, but representative of the problem.

Horsepower has increased even more on average - around 107 percent. Automakers have managed to increase fuel efficiency by an impressive 15 percent between 1980 and 2006, according to Knittel - but had weight and performance not shot up at such a massive rate, the actual increase in fuel economy would be nearer 60 percent.

That would equate to a fleet-wide average of 37 mpg today, rather than the current 27 mpg average. Part of the reason for the relatively small increase, Knittel finds, is also that more people than ever have been buying larger vehicles, such as trucks and SUVs. In 1980, light trucks made up about 20 percent of passenger vehicle sales. By 2004, that had risen to 51 percent.

Among the more startling figures in the study, known as "Automobiles on Steroids", are the numbers needed to achieve the 54.5 mpg CAFE standards by 2025.

Maintaining current levels of technological advancement, while reducing weight and horsepower by around 25 percent, would make the 54.5 mpg average easy to hit. If manufacturers were somehow able to reduce the weight of modern vehicles to that of 1980s levels, cars would already be hitting 52 mpg as early as 2020.

Today's carbuyers would be quick to point out that they're already getting higher mileage than they were, and greater performance, while surrounded by all the creature comforts they could want. That's before you even consider cars being massively safer than their 1980s equivalents.

Some may point to the EPA numbers in today's Honda CR-Z as evidence for lack of progress in the auto industry, but the study disagrees. We can't help feeling that if we really want cars to become more efficient, the buying public are the ones that really influence the change.

As Knittel points out: "I find little fault with the auto manufacturers, because there has been no incentive to put technologies into overall fuel economy - firms are going to give customers what they want."