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Volume 41,   Issue 1      January 2006


Not So Fast Not So Furious
    Consider these Fuel Efficiency Factors when Purchasing Your Next Glass Rack Truck

by Steve Brown

Reducing the cost of delivering glass to the end user is of the utmost importance. When it comes to trucks, though, what is the most fuel-efficient means of doing so? While our company is not an expert on trucks (we leave that to the truck manufacturing engineers and the customers), our intent is to provide information that has been shared with us by our customers and the representatives of some truck leasing companies in the United States.

A Good Thing Better

The big debate in fuel efficiency involves the weight of the vehicle versus aerodynamic air drag. The heavier the weight of the truck and payload, the more horsepower (HP) it takes to keep it moving. More HP requires more fuel. However, the consensus among our customers seems to indicate that aerodynamic drag has more effect on fuel mileage than weight for both smaller cab/chassis and large trucks. As vehicle speed increases, the force required to overcome aerodynamic drag and the rolling friction increases. The rate of increase in aerodynamic drag with increasing vehicle speed is much greater than that for rolling friction. 

Air interference caused from other vehicles, climate effects and road conditions also influence aerodynamic drag. For the most part, the trucking industry has done a good job of developing forward-facing surfaces that are aerodynamically designed to reduce air drag. 

The bodies and trailers must also be considered when purchasing a glass rack. Deflectors that that will direct the airflow away from the body or trailer front face and reduce aerodynamic drag can be added to any truck body. The amount of reduction in air drag is always influenced by the driver’s driving habits, the climate conditions and, most importantly, the speed of the vehicle. 

Small vs. Large


On small trucks (3/4 – 1 ton) Ford and GM representatives say wind drag is a more important factor to fuel efficiency than weight, and our findings tend to support these claims. We simply reshaped the forward front of one of our enclosed auto glass bodies by building 45-degree angles on the front corners. This one change increased the mileage of the truck by almost 2.0 miles per gallon. 

Diesel engines can also increase fuel efficiencies, but the additional cost of the diesel engine along with the increased cost of diesel fuel when compared to gasoline (at press time) must be considered as well. 

The large trucks used for long (more than 20 feet) glass rack bodies and trailers are more difficult to study. The glass rack is affected by air turbulence due to head winds, crosswinds and wake flow behind the body. The effect of this air turbulence also is affected by the vehicle’s velocity. It is important to note that the shape of the glass rack is driven by cargo space. 

When aerodynamic design changes are made to improve fuel efficiencies on these large glass racks, bodies and trailers, the driver’s driving habits, maintenance of the vehicle, tire pressure, safety of the design, weight and overall cost of the design change need to be considered as well. The way the glass is loaded and unloaded is also important. Loads should be aerodynamically placed with stability and safety well planned. 

Wind Effects

The affect of air turbulence from headwinds, crosswinds and wake flow is significant to the fuel efficiency of glass racks, as well. Our company believes the impact of these winds may be more significant on “open” bodies than “closed” bodies. By sheeting the exterior A’s on a body or installing soft sides on trailers, installing a roof with a rear door, attaching a deflector on the front of the body and possibly sheeting the rear under-carriage, the effect of air turbulence on the glass rack body or trailer may be lessened, which, in turn, should increase fuel efficiencies. 

Automatic vs. Manual

The choice between an automatic transmission and a manual transmission can also be a deciding factor.

One truck-leasing representative told us that automatic transmissions on the larger trucks generate a one-miles per gallon improvement over a manual transmission since the shifting takes place automatically. He told us today’s electronic transmissions are matched up to the electronic engines and computers select the optimum shifting timing. 

When it comes to fuel efficiency and cost reduction, you will have to decide for yourself what is best. But consider this information that was shared with us by a leasing representative of one of our customers:

“Our data … supports [the conclusion] that the speed the vehicle travels at and the driver’s driving habits, has the most influence on miles per gallon. For every one mile per hour (MPH of speed) over 55 MPH, you will lose 1/10 of a mile per gallon. So, at 65 MPH, you are sacrificing a full one mile per gallon of fuel. This may not sound like much until you put the pencil to it at today’s fuel prices. [Consider this] example: 65,000 miles per year at 8.5 miles per gallon vs. 7.5 equals a savings of 1,020 gallons x $2.50 per gallon = $2,550 per vehicle per year.” 

Take the $2,550 times the number of trucks in your fleet and see how much savings your company could realize annually by simply slowing down the speed of your vehicles. 


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