If you are in the market for a mid-size car, what to pick? We looked at nine mid-size, average-priced cars available for sale in the U.S. The factors we all consider before buying are price, interior room, exterior appearance, trunk size, economy, power, refinement, acceleration, safety, handling and that feeling in your gut that you just like the idea of owning and driving a particular model.

Economy is today’s big concern when it comes to buying a car.

We looked at the Honda Accord, Toyota Camry, Nissan Altima, Chevrolet Malibu, Ford Fusion, Mitsubishi Galant, Mazda 3 and Mazda 6, Volkswagen Passat and the Hyundai Sonata, all cars in a similar price range except for the Hyundai Sonata, which has a $2,000 to $4,000 advantage over its competitors, and the Passat, which is $6,000 to $8,000 more expensive than its competitors.

Fuel consumption is determined largely by three factors: the weight of the car, the “aerodynamic” slipperiness or efficiency of the car’s shape and the basic efficiency of the engine and transmission.

Weight affects fuel consumption mostly during acceleration and when climbing hills, and therefore has most effect in the city. “Aerodynamic” efficiency affects fuel consumption over 50-55 mph, therefore has the most effect on highway driving.

Engine efficiency affects both city and highway driving, and transmission efficiency, at least in modern transmissions, affects city driving more than highway driving. The more speeds available in a transmission, the more efficient it is likely to be. For example, a six-speed automatic transmission is more efficient than a four-speed or a five-speed, and a continuously variable, no-shift transmission should be the most efficient.

Dual overhead camshaft engines are generally more efficient than single overhead camshaft engines, which in turn are usually more efficient than overhead valve engines. Variable valve timing is more efficient than fixed valve timing, and 4-cylinder engines, although less powerful, are usually more efficient than 6-cylinder engines. A larger, higher horsepower engine will use more fuel than a lower horsepower engine in the same car because the driver will use the extra performance, and will pay for it by using more energy/fuel.

Modern mid-size cars all offer 4-cylinder (2.4/2.5 liter) models for economy and 6-cylinder (3.5/3.8 liter, except for Ford Fusion at 3.0 liter) models for more power and acceleration. Some manufacturers only offer the best luxury features and options in the 6-cylinder model. Others offer the same features and options in 4-cylinder and 6-cylinder models.

What did we find? The EPA says the most fuel miserly of the 4-cylinder, mid-size cars in city driving is the 2009 Nissan Altima at 23 mpg, probably because it is about 200 pounds lighter than its competition and has a continuously variable transmission (CVT). The best on the highway of the 4-cylinder cars is the Chevrolet Malibu at 33 mpg, followed by the Hyundai Sonata at 32 mpg.

In the 6-cylinder cars, the Accord, the Camry, the Altima and the Sonata are best in city driving at 19 mpg. On the highway, the Accord and the Sonata rule with 29 mpg.

The fuel consumption losers in this review are the VW Passat 4-cylinder at 19 city/29 highway and 6-cylinder at 17 city/26 highway; the Mitsubishi Galant 4-cylinder at 20 city/27 highway and 6-cylinder at 16 city/25 highway; and the Mazda 6-cylinder at 17 city/25 highway.

All other things being equal, and your driving is a roughly equal mix of city and highway, the ultimate choice for average fuel consumption in the mid-size sedan market is the 4-cylinder Chevrolet Malibu with 27.5 mpg followed by the 4-cylinder Nissan Altima and the 4-cylinder Hyundai Sonata with 27 mpg. If you have to have a 6-cylinder version, the ultimate choice is the Honda Accord or the Hyundai Sonata, both with a 24 mpg average.

What? The Sonata shows up in all categories, and it’s cheaper? Hmmmm. And the 4-cylinder highway and average champ is a Chevrolet Malibu? How about that Altima? Accord and Camry, watch out, there’s some good competition snapping at your heels.

Motoring since 1952, Austin Lynas has owned 59 cars and at one time raced mini-sedans in England. An aerospace engineer, he was instrumental in developing the anti-skid systems used in British military aircraft that preceded today’s ABS systems. Lynas can be reached by e-mail at austin@sandiegometro.com.