If you are in the market for a used semi truck, you need to take fuel economy into account. Although it's impossible to get a semi truck that is completely fuel efficient, there are attributes that you can look for that will save you money in the long run. While you're shopping, look for these features:

1. Trailer Tails

When shopping for used semi trucks, look for ones with rear tail fairings or trailer tails hanging off the back end of their trailers. These end panels have been around since the 1930s, making it possible to find them on used trucks, but unfortunately, it may be difficult as they have only become popular lately. Luckily, if you cannot find a used semi with tail fairings, you may be able to retrofit the trailer tails.

Designed for long haul drivers, these panels are designed to reduce drag, and as a result, they work the best when you are driving at relatively fast speeds. If you typically drive your semi in towns or cities, trailer tails may not be important to you. However, if you usually run your semi over 65 mph, you should insist upon them. In that context, they can help drop your fuel bill by 5 to 15 percent.

2. Side Skirts

Just as trailer tails help to make a used semi more efficient, so too can side skirts. Side skirts hang down along the side of the semi trailer, and they reduce the amount of air that sweeps under it. As a result, there is less wind resistance under the trailer, and you don't have to fight through it and use up fuel.

3. Small Gap Between Cab and Trailer

Air can also get stuck between the cab and the trailer, and the resistance from this air can cause your truck to require more power to move forward. As a result, your semi has to use more gas. However, if you can find a used semi with a very small gap between the cab and the trailer, you will enjoy slightly improved fuel efficiency.

4. Super Single Wide Wheels

Traditionally, semis have four double sets of wheels, but if you want to boost your fuel efficiency, you should look for super single wide wheels. According to one study, using wide double wheels reduce a truck's fuel consumption by 2.9 percent.

That may not sound like a lot, but if you are using your truck year round to transport things, you are likely doing an average of 125,000 miles per year, and reducing your fuel consumption by 3 percent is likely to save over 700 gallons of gas.  

5. Even Tread on Tires

Whether you opt for single wide or traditional double tires, look at the treads to make sure they are evenly worn. If you buy a used semi that has tires with uneven tread, it may reduce your fuel efficiency. However, if you cannot find tires with even tread, you should talk with the used semi sales associate about replacing the existing tires with new ones.

6. Aerodynamic Curves

The most fuel efficient semis to hit the market recently have one big thing in common -- they all have a very aerodynamic design. This is achieved by a curved front end and cab and a close fit between the cab and trailer.

Even if you don't have enough money to buy a brand new fuel efficient semi, you can take inspiration from those models, and you can look for a used semi that has the most aerodynamic shape possible.

For more tips on finding the most fuel efficient semi possible, talk with a used semi dealer such as Arrow Truck Sales.