When you're shopping for a fuel-efficient car or truck, common sense suggests the smallest available engine delivers the highest mpg. In the real world, that's not always the case. A powerplant that strains, wheezing out inadequate horsepower and torque for the job, just might send you to the gas pump more often rather than less often. To say nothing of the fact the life of an overworked engine is not usually a long one. So while a 4-cylinder engine tends to be more economical than a V-6 powering the same car, and a V-6 is more frugal than a V-8, smaller isn't always the wisest choice.

What's needed is the best match between car size/weight and engine output. Too small, and it's often overworked, never realizing its economy potential. Too big, and it guzzles more than necessary to get the job done. To choose between a standard and optional engine, check the EPA ratings and the real-world road tests -- not only for mileage figures but for comments on the sufficiency or lack of usable power.

Turbochargers and Superchargers

At first glance, a turbo sounds like the high-efficiency choice for both performance and economy. After all, it doesn't drain engine power, but makes use of exhaust gases to rotate the high-speed turbine. Better yet, it comes into play only when needed-only when tromping hard on the gas pedal for a quick burst of extra power. That extra jolt sucks up plenty of extra fuel, however, as it shoves an oversized air/fuel charge into the engine. If rarely used, it might not hurt mileage much. But how many people buy a turbo and keep their foot light on the pedal? Superchargers, driven directly by the engine, act as a constant drag and cost a bundle in mileage.

Choose an Economical Axle Ratio

Plenty of buyers never realize there's a choice. Often there isn't; but many pickup trucks and some performance-oriented cars are offered with a selection of ratios. As a rule, the lower the number, the greater its economy potential. That's because it allows the engine to run slower for a given road speed. An "economy" axle has a ratio below 3:1 or so. "Performance" axles, which deliver quicker acceleration and are better suited to towing trailers, might come to more that 4:1. The perfect selection depends on the type of driving you do.

Shift for Yourself

A quick glance at the EPA ratings for cars available with a choice of manual and automatic transmissions makes it clear that manual gearboxes are the only choice for peak economy. Seldom does the city-driving estimate for automatic come closer than 2 to 3 mpg to the manual-shift figure. In some cases, the difference is similar on the highway; other automatics achieve better results, rivaling a "stick" when up to speed. Compare the figures before deciding, but remember that high mpg wi th a manual comes only when it's shifted with some expertise.

All-Wheel Drive

Many cars and minivans are available with all-wheel drive (AWD). The AWD systems in cars and minivans is intended as an all-weather traction aid, and not designed for off-road duty. Thus, it doesn't have the weighty, heavy-duty componentry of most four-wheel drive (4WD) and AWD systems in pickup trucks and SUVs.

AWD cars and minivans do tend to use more fuel than their 2-wheel drive counterparts. This is due less to any added drag placed on the powertrain by AWD as more to the 100 to 200 pounds the AWD system adds to the weight of the vehicle. But the fuel-mileage difference isn't pronounced, and while AWD adds to the purchase price of the vehicle, it's well worth considering if you frequently travel wet or snowy roads.

Amenities and Fuel Economy

To some people, comfort is a heated leather seat in the winter. Others take comfort in knowing they're eking out every last mile from each gallon of gas they consume.

Nearly every luxury amenity adds weight and drains power, both of which are the enemies of fuel economy-and performance. You'll either drag down the efficiency of your engine, or have to shell out for a larger, less efficient engine designed to shrug off the extra strain placed on it by the following power convenience features.

Air conditioning: Air conditioning is standard on all but a few low-cost compact cars and trucks. It's a necessity in many parts of the country. And even when the weather isn't sweltering, the ability to drive with windows closed can reduce driver fatigue on long trips or in noisy city traffic. Still, an air conditioner adds a hundred pounds or so to the car's weight and drains energy even if the switch is never flipped on. In the city, you're talking about as much as 3 to 4 lost mpg whenever it's used. Can you learn to live without it?

Sunroofs: Just like an open window, an open sunroof adds drag to the car's ability to slice through the air. And a sunroof's sliding glass or metal panel, electric motor, and the tracks and reinforcements upon which it rides to open and close all add lots of extra weight to your vehicle.

Cruise control: Cruise control can boost mileage if it's used properly on long, flat stretches; but can drain if operated carelessly. If you do plenty of highway driving, it may be worth the price in both economy and convenience.

Roof rack: Is it really worth hauling a wind catcher all year long just to have it available during vacation time? If so, try to avoid putting too much bulky stuff up there. An older, non-aero sedan or wagon might not be affected as much as a modern vehicle, in terms of mileage.

Colors: Light colors reflect sunlight and help keep the interior cool. Dark colors do the opposite. Color choice, then, affects the need to use the air conditioner or heater.

Power seats, windows, door locks: Handy, yes; economical, no. Each accessory draws energy or adds weight, decreasing mileage.

Before you buy a fuel-efficient car, consider which of these options is really necessary to you, and weigh each option against the fuel economy you'll sacrifice.