Perhaps you've noticed the phenomenon of the incredible shrinking car. Yo-yoing gas prices and a stumbling economy have had a visible downsizing effect on the types of vehicles Americans are choosing to buy these days. Small cars are simply cheaper to run than bigger ones, and no one believes that this price reprieve at the pumps will last very long. Now that there is a demand for small cars, however, we need to find the cars to fill it. For that, we need to turn to our more fuel-efficient neighbors across the ponds for help.

Do you think that Americans can accept smaller, more fuel-efficient rides?

According to a recent study from Jato Dynamics, a leading provider of automotive business intelligence, U.S. cars emit 85 percent more carbon dioxide and consume twice as much fuel as European and Japanese cars. The average year-to-date fuel consumption figure for cars, minivans and SUVs in the U.S. market stands at 22.6 mpg, compared with 40.3 mpg in Europe and 40.6 mpg in Japan. Those are staggering differences, and they make us wonder why it will take American automakers until 2020 to reach a mandated average fuel-efficiency rating of 35 mpg, when European and Japanese automakers already do. (That's food for thought, folks.)

But we digress. This article is about vehicles that get 40 mpg or better on the road. Sadly, only two are currently available in the U.S. Here's the top 10:

1. Toyota Prius

Click to enlarge picture2009 Toyota Prius (© Toyota Motor Sales, U.S.A.)

The Toyota Prius is still the reigning mileage champ in the U.S., earning an EPA rating of 45 mpg highway, 48 mpg city for 2009.

The Toyota Prius is the reigning champion for fuel efficiency in the U.S., and has been since 2007, when the Honda Insight hybrid was put to sleep. The Environmental Protection Agency rates the 2009 model at 45 mpg highway, 48 mpg city. This kind of efficiency is achieved by pairing up a gasoline engine with an electric motor and a large storage battery. In addition to saving you lots of dough at the pumps, the Prius ranks as one of Consumer Reports' most reliable cars. In fact, three of the six most reliable family cars are hybrids (the Prius, Camry and Nissan Altima).

Can Americans buy it? Yes.

2. Honda Civic Hybrid

Click to enlarge picture2009 Honda Civic Hybrid (© American Honda Motor Co., Inc.)

The gas-electric Civic delivers 40 mpg in the city and 45 mpg on the highway.

Like the Prius, the hybrid Honda Civic has an electric motor that shares the work of the gasoline engine. Though not quite as miserly as the Prius, the gas-electric Civic still reaps a respectable 40 mpg in the city and 45 mpg on the highway, according to EPA tests.

Can Americans buy it? Yes

3. Ford Fiesta ECOnetic

Click to enlarge pictureFord ECOnetic Fiesta (© Ford Motor Company)

Ford's ECOnetic Fiesta packs a 1.6-liter Duratorque turbodiesel engine that offers more than 60 mpg.

If you were to drive Ford's snappy little Fiesta ECOnetic, you might easily find yourself spending less for fuel each week than you do for coffee. It employs a clean, efficient 1.6-liter Duratorque turbodiesel motor and low-drag body design to achieve a combined city/highway fuel-economy rating of 63.6 mpg, based on European Union standards.

Can Americans buy it? No. Ford says that importing the Fiesta ECOnetic from Europe, where it's made, would be too pricey and wouldn't be met with enough demand. Modern diesel engines deliver roughly 30 to 35 percent higher fuel efficiency than gas-powered engines, and are often just as clean as hybrids. The problem is that diesels must meet the most stringent emissions standards in the world to be sold here in the U.S., and thus automakers are reluctant to go through the costly process of modifying them to be greener and bringing them into a market that is simply lukewarm on diesel power. Folks, diesel is no longer the dirty bird of fossil fuels. We need to give them a closer look.

4. Mercedes-Benz C 250 CDI

Click to enlarge pictureMercedes-Benz C250 (© Chrysler LLC)

The Mercedes-Benz C 250, part of the BlueEfficiency diesel line, offers up 45 mpg and a top speed of 155 mph.

Getting past the 40 mpg mark doesn't have to leave you yawning, as evidenced by the Mercedes-Benz C250 BlueEfficiency. According to Mercedes, the C250 can get 45 miles from a gallon of diesel and has a top speed of 155 mph. It also features a slick, luxurious interior and an impressive features package. In addition to the specially designed 2.2-liter diesel powerplant, the C250 BlueEfficiency boosts its mpg with features such as an electric power-steering motor that deactivates when not needed, and a 7-speed automatic transmission that decouples when the car is in idle.

Can Americans buy it? No. It's a diesel issue.

5. Smart fortwo

Click to enlarge picturesmart fortwo CDI (© Chrysler LLC)

The fortwo CDI makes the efficient smart car even smarter. The diesel version delivers more than 70 mpg.

As tiny as it is, the smart fortwo available in the U.S. earns an EPA combined fuel economy of only 36 mpg. Over the Canadian border and across the pond in Europe, smarts are sipping diesel fuel to the tune of 70 mpg.

Can Americans buy it? We're afraid not. The diesel fortwo doesn't yet meet U.S. emissions standards. Passionate patriots have struggled unsuccessfully to bring these diesel-powered mites down from the Great White North, and saddened Canadians have had to leave theirs behind at the border. To make American green-car enthusiasts even more jealous, smart is beginning the release of an electric fortwo in Germany. Learn to love diesels, folks. Lawmakers should look closely at this list, too, when they are deciding how to reduce our carbon footprint.