A mandate to reduce greenhouse gas emissions by requiring 53-foot trailers pulled by truckers to enhance gas mileage may backfire.

Three Manteca-Lathrop trucking firms contacted by the Bulletin are critical of the requirement for trailer skirting noting among other things:

•There is no proven scientific study that shows retrofitting the trailers with the plastic skirting that typically costs $1,500 per truck is effective under driving restrictions imposed by the state.

•Manufacturers of skirting note it is most effective at speed of 62 mph when the speed limit for trucks in California is 55 mph.

•The fairly low clearance that makes the skirting prone to damage at many railroad crossings.

•Concerns of how the skirting would impact safety in an accident.

One trucking firm - AC Trucking - has one truck retrofitted with the skirting and is reporting the opposite of what the California Air Resources Board said would happen: Mileage worsened.

“We run primarily north-south,” said AC Trucking owner Al Nunes. “The prevailing wind patterns in the valley are east-west.”

That means the skirting has expanded the surface area where side winds would increase drag and therefore lower mileage.

The objective is to improve mileage in order to reduce fuel consumption which in turn reduces emissions.

CARB exempts a number of types of trailers and tractors from the skirting requirement including box-style trailers less than 53 feet as well as cargo containers transferred to and from intermodal rail facilities seaports. Fire trucks, solid waste trucks, and military tactical vehicles are exempt from the skirt rule as well.

The skirting requirement is the outgrowth of Assembly Bill 32. It is the same law that many cities believe will substantially increase the cost of doing business in California and especially the Central Valley that is under the gun for not meeting ever tightening federal air quality standards.

Area elected leaders - including the Manteca City Council - view Assembly Bill 32 implementation plans as being the ultimate jobs killer. The League of California Cities has questioned the wisdom of many of the Assembly Bill 32 rules that are being developed.

The greenhouse reduction rules impact all segments of the economy from farming to manufacturing.

Manteca-Lathrop is considered a major trucking hub due to the growing number of distribution centers. Truck traffic in the area also will increase once the Union Pacific nearly triples its intermodal facility in Lathrop as part of a plan to shift a large chunk of its operations from the Port of Oakland.

A combination of side skirting on trailers and low-roiling resistance tires are expected to meet the new CARB rules for fuel savings that are going into effect.

Trailer skirts have been found by some manufactures to save almost 7.5 percent in fuel use at 62 mph. That is optimum performance. It is also 7 mph faster than California’s top legal speed limits for trucks.

That is in addition to cleaner burning diesel engines that have been required adding five figures to the cost of buying - or retrofitting - a truck tractor. California truckers already use a low-sulfur fuel that is lower that used in the 49 other states.