Fuel efficiency is derived from a cars basic characteristics, including aerodynamics, weight, engine size, maintenance, and the driver's habits.
Over the years, a number of vehicle designs have been created that defy what seem like evident rules of efficiency. The Nissan Cube and similar vehicles, for example, present an unconventional and aerodynamically intrusive shape that nevertheless nets most vehicles styled in this fashion about 30 miles per gallon. The reason for this is because the aspect of fuel efficiency most commonly associated with aesthetics is the drag coefficient. Designers know this, too, and wind tunnel testing is an almost universal procedure during vehicle design. As a consequence of this focus upon aerodynamics, there is very little variation in drag coefficient of passenger vehicles. The hyper efficient Prius and Insight models both have drag coefficients of about .25 depending on the model, year, and the amount of bug entrails dried upon the hood. Most passenger cars fall somewhere between a .3 and .4 coefficient of drag.
What does the drag coefficient matter? This is one of a number of contributing factors to fuel efficiency, although it is often over-attributed. Other factors include vehicle weight, tire resistance, engine size, and drive train efficiency. The rolling tire resistance, power used by the car's systems, and drag coefficient all yield an equation known as road load, which is expressed as av + bv2 + cv3. Essentially, this equation helps decide where the balance point occurs for optimum mileage. What you need to know is that this balance point occurs somewhere shortly after you reach your highest (cruising) gear. So, the most efficient speed is somewhere between 40 and 60 miles per hour, on average. It is higher for smaller vehicles, but the friction and air resistance becomes prohibitive much more quickly for larger vehicles. The reason that aerodynamics play less of a factor than one would suppose is that wind resistance increases with speed. While aerodynamics will play the largest role at freeway speeds of 70-80 miles per hour, they are much more important at race car speeds of 200 miles per hours or more, or for vehicles specifically designed for fuel efficiency (note the Nuna 5 World Solar Challenge Winner below). The Nuna5 holds the world record drag coefficient at an incredible .07, and passing raindrops probably glare at it with envy. Aound town, however, aerodynamic effects are marginal at speeds of 50 and below.
For ordinary passenger vehicles, however, cargo and passenger space is important, and each of these considerations causes a wider profile and consequently a lower drag coefficient. Still, these vehicles retain a relatively sleek design that downplays the effect of air resistance. This leaves the remaining factors of engine size, weight, component friction, and driving/maintenance habits.
With a very good correlation, you can generally assume that a smaller engine implies greater fuel efficiency than a larger one, whether in the number of cylinders or the overall displacement and cubic-inch size of the motor. There are exceptions; for example, the 3-cylinder motors of Geo Metros were gradually beaten out by the development of efficient 4-cylinder engines in the Civic, Camry, and Accord. Generally, however, you can make the assumption that a smaller engine is more efficient. It takes less fuel to run the engine at idle, and it usually takes less to run it during acceleration, deceleration, and cruising phases than a larger counterpart. Larger engines also add to weight, which increases rolling resistance.
Vehicles with narrower tires have less grip and traction, but they also have less surface area of the tire tread in contact with the road at any given time. This decreases the resistance of the tires, as does a lower overall weight of the vehicle mentioned previously. Rolling resistance is similar to air resistance in that it increases with speed.
Two final factors regarding fuel efficiency are driving habits and maintenance quality. If you love accelerating rapidly from an intersection or stop sign, you are murdering your gas mileage. In fact, fuel efficiency variations between vehicles of the same make and model can largely be explained by an analysis of driving patterns (in addition to vehicle maintenance). Any driving action that implies you are in a rush such as following too closely, passing, constantly having your foot on either the gas or the brake, or driving rapidly does terrible things to a car's mileage. Alleviating this is simple: just change to become a slow and deliberate driver. Also, regular oil changes and following the service schedule of your vehicle will extend its life and efficiency.
So, if you are selecting a car for fuel efficiency, then you should compare and consider the vehicle's attributes. Its potential miles per gallon is a factor of the information listed above. The evidence makes it plain to see why SUV's and trucks are not fuel efficient: they are the losers of every category.