Whether you buy a new or used vehicle, fuel efficiency--good gas mileage--is high on the list of most buyers' concerns. The difference between choosing a fuel-efficient car or one that guzzles gas, will either save or cost you money over the life of the vehicle, which could be substantial. Fuel efficiency varies widely from one car to the next. Obviously you can check the EPA rating for city/highway MPG on the window sticker, although most of us know the average car never reaches those numbers.

You can also check consumer guides, car magazines and Web sites, Web site forums or ask friends, relatives and co-workers which vehicles they recommend as fuel-efficient cars. Don't buy more car than you need, as larger vehicles generally have bigger engines that are less fuel-efficient. Find the most fuel-efficient car in the size group you're interested in, whether a two-seater, compact, mid-sized, SUV or pickup truck. There are several online sites where you can compare fuel consumption ratings of any car.

Your choice of transmission can also affect the fuel efficiency of the vehicle. Generally, a car with manual transmission is a more fuel-efficient car than one with automatic-assuming you shift properly. And a manual with overdrive, tachometer or shift indicator is the biggest fuel saver, saving up to 10% on fuel costs. If you do buy an automatic, which makes more sense for larger cars, the more gears the better.

Under normal driving conditions, smaller engines offer better fuel efficiency and economy than larger ones. All other things being equal, the larger engine and the more cylinders it has, the more fuel it consumes. Additionally, cars with smaller engines usually cost less and gas costs are lower because you don't need higher octane gas. That doesn't mean a bigger engine is never a good choice. In some cases, a larger, more powerful engine may provide the greater fuel efficiency. If you use your vehicle for work or often tow heavy loads, a smaller engine could burn more fuel if it has to work too hard and function beyond its most fuel-efficient range.

Depending on the type and size of motor vehicle you purchase, you may have the choice of front-wheel, rear-wheel, four-wheel or all-wheel drive. The majority of passenger cars and minivans have front-wheel drive, a design that supplies better traction and more interior room than rear-wheel drive. Although front-wheel drive was originally adopted to improve fuel economy over rear-wheel drive by reducing the weight and size of cars without giving up driving performance or interior space, there's really not much difference in fuel efficiency between the two.

And although four-wheel and all-wheel drive provide better traction and braking in certain driving conditions, the weight and friction of the additional drivetrain parts may increase fuel consumption by up to 10% over a two-wheel drive vehicle. Most often in SUVs and pickup trucks, four-wheel drive is enabled at will by the driver when additional traction is necessary. All-wheel drive is an option on some SUVs and a minority of passenger cars. Full-time all-wheel drive, however, makes for the least fuel-efficient car, because all four wheels are always being driven, drawing power from the engine and thus using more gas.

Another way to be a fuel saver, is by limiting the options you select for your car. You may not have realized that many conveniences from power windows, seats and mirrors to air conditioning and seat warmers decrease fuel efficiency and cost you more in fuel consumption. They add either weight, increase aerodynamic drag or pull extra power from the engine or through the alternator. Aluminium wheels are one of the few options that actually reduce weight and thereby increase fuel efficiency.

Obviously, considering other fuel options such as a hybrid electric car, is another choice for a fuel-efficient car.