The giant, bold-faced 40 MPG billboard greeted drivers leaving the Queens Midtown Tunnel entering New York's Long Island Expressway for months: the Hyundai Elantra, proudly displaying its new sweet spot for consumers seeking fuel economy.

As gas prices have climbed over the last four years, to a national average highs of $3.29 per gallon this holiday season, the pinch of filling up a gas tank has become particularly excruciating.

Fortunately car makers have largely answered the call, adding compact, fuel-efficient cars to their fleets. The numbers were once unthinkable. Hyundai's much-vaunted 40 mpg? Ten cars on the market within the 2012 model year can lay claim to the same number or higher on city mileage, six in highway mileage.

But in the months since, Hyundai's 40 mpg ads have become fewer and farther between along highways. Because for all the bluster about increasing fuel economy, there's a disparity between EPA estimates and real-life results. In some cases, the differences can be negligible. In other cases, the reality does not match the hype.

It's a bit too easy to blame carmakers for the difference. The truth is, they're married to a system that's anachronistic, a set of regulations in desperate need of an update, and a mixed consumer market that sees gas-guzzlers make up three of the top five best selling cars of 2011.

Some changes are on the way, but they will not prevent an 11 mpg discrepancy, which the South Korean-made 2012 Elantra suffers from, according to reports from drivers at fueleconomy.gov.

Hyundai's problems started when advocacy group Consumer Watchdog wrote a letter to the U.S. Environmental Protection Agency, lambasting the carmaker's 40 mpg claims for the popular Elantra.

The Elantra has attracted an unusual number of consumer complaints about real-world mpg averaging in the mid-20s, far from Hyundai's stated average of 33, it said.

Hyundai responded to the complaint, pointing to 39-mpg results after Consumer Reports' testing. The car also outshone its competition in city driving tests.

In testing conducted by Motor Trend, the Elantra averaged 29 mpg. USA Today found a very pedestrian 22 mpg. The less than advertised charge especially smarts during the end-of-year sales push. Car makers use the holidays to peddle their best-selling models, lavishing red bows atop their cars as doting couples stare out into a snowy pastiche ad nauseam during commercial breaks. Their luck! A comfortable, low-maintenance car with negligible cost to operate!

For the two most recent model years, Hyundai Motors has actively marketed its base models of the Elantra on their very high 29/40 MPG, and 33 MPG average, leaving a trail of disappointed drivers, Consumer Watchdog wrote.

The truth is, neither Hyundai nor Consumer Watchdog is to blame. Any discrepancy between advertised and actual gas mileage points back to a dated system that has aged gracelessy over five decades.

EPA mileage estimates are self-certified, with the agency randomly testing a small percentage of a carmaker's fleet of models any given year.

The means of testing a car's gas mileage, however, is uniform and has been for decades. A recent change in EPA testing has sought to add more reality to the testing, with varying degrees of success.

Realistic Driving Simulations? 

In a process reminiscent of a scene from Ferris Bueller's Day Off (1986), the car is put on the automotive equivalent of a treadmill called a dynamometer, then through a series of accelerations, sustained speeds and stops made to mimic real driving conditions. Throughout the process, a hose connected to the exhaust pipe measures the carbon exhaust, giving the estimate for the amount of fuel used.

The system, though rather dated, is fairly accurate and uniform. The difficulties come in the driving simulations in place. The standards for city and highway driving were set about four decades ago, said David Friedman, deputy director of clean vehicles at the Union of Concerned Scientists.

The driving cycle that they use [...] was developed back in the 1960s and 1970s, when 0-60 in under 10 seconds was a sports car; now it's a family car, he said.

The test calls for acceleration of 0 to 60 miles per hour in 18 seconds, and a top speed of 60 mph. It harkens back to an era when air conditioners were a luxury and fishtail taillights were the norm. Today, even low-grade performance vehicles are blistering by comparison, and large portions of the nation's highway system have a speed limit set significantly higher.

EPA knows this, they know it very well, Friedman said. So they've always discounted the window stickers.

The mpg figures that result from the tests are used to make sure cars meet Corporate Average Fuel Economy (CAFE) standards, and are decreased by factoring in several conditions before becoming the figure on the dealership's sticker.

Consumers had little to go by when determining gas mileage for their purchase. (Though years of low fuel costs made gas guzzling SUVs the norm in the 1990s, and rendered mpg meaningless to many buyers). But with fuel consumption becoming a greater concern for economic and foreign policy reasons, the EPA proposed revising its testing methods in 2006.

The classic test was given three additional phases, meant to reflect gas consumption under more accurate driving conditions. High speed, air conditioning and cold-temperature driving were added to the battery of tests, and are factored into the ultimate mpg figures shown on new EPA labels rolled out this year. They will become the norm in 2012.

The new stickers post a car's combined city/highway average, as well as a dollar figure on gas savings, annual fuel costs and eco-friendliness ratings. Friedman welcomed the stickers, saying the new labels can only help.

You want the consumers to have the best information possible, he said. Consumers have to have good information to make good choices.

The window stickers will never be accurate for everyone. If you drive in a city, or live in a rural area, you're going to get a different fuel economy.

It's the same sort of sticker posted on new Hyundai Elantras and other compact cars that promise to sip gas. Still, a new label does little to close the gap between the actual mileage and the sticker value. For that, Friedman suggests the EPA revisit its tests to see they are effective.

The next step -- EPA needs to go back and collect a lot of data to make sure their new process is actually working, he said. What I would love to see is within the next five years Congress to take this up again.