When it comes to fuel-efficient cars, there's often an assumption that the smaller a ride is, the better its gas mileage. That may not be the case, some experts say, with subcompact cars sometimes not faring any better than the larger, more spacious, compact models, CNNMoney reports.

To illustrate the point, consumers need look no further than the Chevrolet Sonic. The tiny car gets a respectable 33 miles per gallon, and while that's certainly impressive, it's the same as the larger Chevrolet Cruze. The small Hyundai Accent also gets the same mileage (33 mpg) as the bigger Hyundai Elantra.

The biggest reason for the phenomena is that cars can only get so aerodynamic. Vehicles have to be a certain height for passengers to fit comfortably, as a result the shape of subcompact cars often is not as aerodynamic as it could be, and  the gas mileage suffers.

"You can shorten it up and make it narrower, but the height of a car can only be so small," Scott Miller, director of mass, energy and aerodynamics at General Motors, told CNN.

Since size is not the only factor to consider when looking for a fuel-efficient used car, and you should take a few other things into consideration, experts say. For instance, there are certain amenities that may cost you some mpgs later on. One of the biggest, some say, is a sunroof. An open sunroof creates more drag and could lower a car's fuel economy.

Anybody looking to save money not only on gas, but also on their used car, should head to New Jersey State Auto Auction. With a wide variety of makes and models, including everything from subcompacts to SUVs, there's something for everyone.