Toyota PriusClick to enlarge picture

The most fuel-efficient passenger vehicle in the U.S. is Toyota's Prius gasoline-electric hybrid car, with government fuel economy ratings of 60 mpg on the highway and 51 mpg in the city.

Maybe gasoline prices are crimping your household budget. Maybe you'd like to reduce the U.S. dollars that flow to the Middle East for oil. Perhaps you're motivated by concern for the environment, or the nagging reality that oil is a depleting resource that shouldn't be wasted.

Whatever the reason, many Americans—including you, perhaps—are looking for more fuel-efficient vehicles.

So what should a consumer know when deciding among low-mileage gasoline cars, diesel-powered vehicles and gasoline-electric hybrids?

Fuel Economy Reality
Put into perspective the fuel economy numbers posted on a new vehicle's window sticker. Studies show the average driver has been getting only 75 percent or so of the mileage figures that are on the sticker of their new vehicle.

This will change in the fall of 2007 as the federal government begins phasing in a new way to calculate those fuel economy figures for the window stickers. The goal is to make the fuel economy statistics better reflect what real-world drivers will get.

You see, the numbers aren't derived from real-world driving but from U.S. Environmental Protection Agency emission testing procedures on new vehicles. Thus, the laboratory procedure does not involve cars using gasoline for the testing. Additionally, many variables—including weather, terrain, driving habits and condition of the vehicle—affect the kind of mileage that regular drivers get.

This is not to say the reported numbers can't be used for comparison purposes between vehicles, especially those in the same class.

But some owners of gas-electric hybrids, in particular, have voiced disappointment in the disparity between their mileage and that posted on their vehicle window stickers. These owners of Honda, Toyota, Lexus and Ford hybrid vehicles have ready access to their real-world mileage via graphical displays on the dashboard, which tend to draw driver attention to fuel use and mileage statistics more so than in conventional vehicles.

Consumer Reports magazine, which calculates its own fuel economy stats, noted that its Toyota Prius hybrid test car got 44 miles a gallon in real world driving, not the city/highway rating of 55 mpg that the government reports.

So, yes, while hybrid vehicles generally provide better mileage than like-sized vehicles in their class, drivers should be aware it will take more than just a gasoline-electric powertrain to get the fuel economy they think they've been promised.

Hybrid Pricing
Gasoline-electric hybrid vehicles typically are priced higher than non-hybrid counterparts—anywhere from a couple thousand dollars to several thousand dollars.

For example, the Honda Civic Hybrid has a starting manufacturer's suggested retail price of more than $22,500 for a base, 2007 model. A 2007 Civic LX gasoline-powered sedan with many comparable amenities carries a starting MSRP that is some $4,800 less.

True, the 2007 Civic Hybrid is rated by the EPA at 49 mpg in city driving and 51 mpg on the highway, for a combined 50 mpg. This is 30 percent better than the combined rating of 33 mpg for the gasoline-powered, 2007 LX model.

But even if a driver maximizes his or her fuel savings and gets the full 17-mpg benefit in the Hybrid, he or she would need more than a dozen years of 15,000-mile annual travel before the gasoline savings—calculated with gasoline at approximately $2.40 a gallon—would recoup the $4,800 extra paid for the Hybrid over the traditional Civic LX.

Some of the price difference also may be recouped by a one-time federal tax credit. In the case of the Civic Hybrid, the maximum credit is $2,100 for a 2007 Civic Hybrid.

Additionally, some companies offer money to employees who buy hybrids. For example, the 185,000 nationwide employees of Bank of America Corp. are eligible for a $3,000 reimbursement from their employer if they buy a new gas-electric hybrid vehicle.

Timberland Co., a maker of outdoor shoes and apparel in New Hampshire, offers $3,000 reimbursements for any of its 5,600 employees who purchase a new hybrid vehicle.

Other Hybrid Issues
Auto industry officials project hybrid vehicle prices will come down as the vehicles become more plentiful and there are greater economies of scale.

Indeed, the starting retail price for a 2007 Toyota Prius is $22,175, which is $170 less than the starting retail price for a 2006 Prius. And early in calendar 2007, Toyota was offering some discounted loan and leasing rates on the Prius in certain markets around the country.

Hybrids have attracted buyers in at least a couple states—Virginia and California—because drivers of hybrid vehicles there have been able to use carpool lanes when they travel solo. But early in calendar 2007, California ended distribution of carpool-access stickers for hybrid vehicles because the limit of 85,000 vehicles was reached.

Another issue revolves around the fact that today's hybrids are relatively new. The technology mates an electric motor to a gasoline engine so the electric motor supplements the engine at times. This reduces greenhouse gases as well as optimizes the use of gasoline.

Today's hybrids wouldn't be possible without electronic engine controls that modulate the smooth working of these two systems together and manage power delivery to maximize fuel efficiency. But the sophisticated mixing and matching of the power is not always easy to accomplish, and buyers might find it best to take their hybrids to the local dealer for service, where technicians have received specialized training, rather than a neighborhood garage.

Hybrids store electric energy on board in large battery packs. The packs are warranted for eight to ten years, depending on the manufacturer, but it's uncertain what the cost will be for replacing old battery packs down the road. Current prices are about $3,000. If this price holds in the future, it could make hybrids less attractive as used cars and thus reduce resale values of these vehicles.

But several auto analysts figure that the popularity of hybrids will prompt greater production of battery packs and thus, lower their prices. Time will tell.

Some emergency workers have been concerned that they could face a danger of electric shock when working on a disabled or crashed gas-electric hybrid vehicle. Auto manufacturers assure them that safeguards are in place and that computers on board the vehicles have a series of safety checks that are designed to avert problems. Still, some first responders are undergoing special training to become comfortable in handling hybrid cars.

Some groups have complained that hybrid battery packs are toxic and likely to become troublesome to dispose of. But automakers say current nickel-metal hydride batteries are recyclable.

Diesel Efficiency
Diesel-powered passenger vehicles are another fuel-efficient option.

Popular in Europe, diesel models are limited in their availability in the U.S. because five states—California, New York, Massachusetts, Maine and Vermont—prohibit their sale due to emission restrictions. Note that in contrast, gas-electric hybrid vehicles are sold in all states. Indeed, hybrids are far cleaner in many emission properties than even conventional gasoline vehicles.

Still, diesels are known for getting extra mileage out of every gallon of fuel. They offer better torque than many gasoline engines. And their price differential over gasoline models generally is much smaller than that for hybrids.

For example, Mercedes-Benz's midsize 2007 E320 diesel sedan has a starting MSRP of $51,550, which is only $1,000 more than the starting price for a 2007 E350 gasoline sedan.

Yet the 3.0-liter V6 in the E320 BLUETEC sedan, as the diesel version is called, puts out an amazing 388 lb-ft of torque starting as low as 1600 rpm and compares with the 258 lb-ft of torque starting at 2400 rpm in the 3.5-liter V6 gasoline engine in the E350 sedan.

The diesel E-Class's fuel economy rating is 26/35 mpg, for a combined 30 mpg, and compares with 19/26, for a combined 21 mpg, in the gasoline E350.

Thus, if drivers maximized the diesel's fuel economy and got the 9 extra miles per gallon, it would take less three years to recoup the approximately $1,000 extra cost for the diesel engine in the E-Class if diesel prices were in the neighborhood of $2.40 a gallon.

Other Diesel Matters
Diesel engines are getting cleaner. Thanks to cleaner diesel fuel with lower sulfur content now available in the U.S. and new vehicle technology that filters or traps troubling engine particulate emissions, diesels are becoming cleaner than ever.

In fact, Mercedes touts its Bluetec as the "cleanest diesel in the world" and has joined with Audi, Volkswagen and DaimlerChrysler's Chrysler Group to spread the Bluetec name across other diesel models in the coming years.

Some consumers, however, will object to having a diesel because these cars typically—though not always—are noisier than gasoline-powered vehicles. Some buyers also might object to the telltale odor that is associated with diesel vehicles.

And, not every urban filling station has a diesel pump. In some cases, drivers might need to venture farther from suburban neighborhoods to locate a place to fill up.

Nonetheless, diesel engines have been with us for a long time—the first diesel-powered passenger car was a 1936 Mercedes—and diesel engines are known for their durability. Hence, they're standard fare under the hoods of big semi-trucks.

Gasoline Models Can Shine
Consumers don't need to venture from conventional gasoline models to find fuel-thrifty vehicles. However, they need to focus on small, lightweight vehicles in order to maximize the gas they use.

For example, the 2007 Nissan Sentra with continuously variable transmission as well as the Kia Rio, Honda Fit, Nissan Versa and Toyota Corolla all are gasoline-powered cars with higher combined fuel economy ratings than that of the 2007 E-Class diesel.

Gasoline models with the best fuel economy all have four-cylinder engines, rather than V6s or V8s.

Ann Job is a freelance automotive writer.

In the market for a new car? MSN Autos is pleased to provide you with information and services designed to save you time, money and hassle. Click to research prices and specifications on any new car on the market or click to get a free price quote through MSN Autos' New-Car Buying Service.