BOLT 2592627 - Caterpillar

The basic principles of determining how to conserve fuel have been around a long time. Only when the cost of fuel increased dramatically was there a scramble by component manufacturers and fleet operators to use these principles.
Fuel accounts for 35% to 50% of the cost of operating a truck today. There is no magical wand that will alter this situation-only knowledge.
However, top management, in many cases, believes it is just a matter of their vehicle maintenance director or shop supervisor turning some mystical engine screw to save 10% on fuel costs. Maybe this theory has surfaced because engine fuel consumption is going down. Let's examine the major components of fuel consumption, beyond the engine.
Rolling resistance, air drag, and driver operation are the three sources of fuel demand. Their interrelationship is getting more attention today. The related problems have been there all along.
Looking at data for straight truck (van body) operation, it may be of interest to note that when driving at 35 mph, 50% of the power demand is needed to overcome rolling resistance, and 50% to overcome air drag. This same straight truck at 55 mph needs 30% of its power to overcome rolling resistance and 70% for air drag.
A tractor-trailer traveling at 60 mph has four times the aerodynamic resistance (air drag) as the identical unit traveling at 30 mph. The configuration of the tractor and trailer influences these factors. A cattle trailer has the most air drag, caused by the air coming in and out of the trailer's ventilating holes. The least aerodynamic resistance is found in a flat bed trailer without sides.
Cargo on a flat bed trailer, by its shape, can trap air. Plastic or iron pipe, loaded lengthwise on the trailer bed, traps a lot of air. Putting tarps over the forward ends of the pipe will force the air over or under the trailer, thus avoiding the problem of air being caught within the individual pipes.
Using 100% as the base for aerodynamic resistance or air drag, tests have shown a cabover tractor and reefer trailer have the following drag:
... 80% air drag.... 5% skin friction of the vehicles.... 10% interference drag-mirrors, antenna, and protruding components.... 5% internal drag-engine cooling, cab ventilation.The configuration or contour of the tractor and trailer has a great influence on the aerodynamics of the vehicle. Rounded leading edges of the cab and trailer can reduce air drag 17% when compared to square corners. Tractors alone with rounded corners contribute 13% of the 17% reduction. The use of air shields and gap panels on the tractor can reduce air drag another 15%.
The overall theory of aerodynamics of a tractor-trailer or straight truck is to streamline the surface of the vehicle. A smooth flow of air moving along the outer surfaces produces less drag than air making abrupt changes in direction. Little insignificant things, such as open cab windows, catch moving air and add to the overall air drag or aerodynamic resistance. Protruding attachments like mirrors, air cleaners, and even