Miles/gallon: Newer vs Older Cars

Can anyone explain how the wide big rig tires for the Smart Way program have less rolling resistance than two smaller "dually-style" tires? It seems like there'd be more resistance due to the larger contact patch. Any truckers on this site?
I think it has more to do with the super single setup weighing less than the traditional dual wheels, 10 wheels and tires vs 18, less weight = better mileage BUT the rig is still going to weigh as close to #80000 as possible to stay legal while hauling the most material down the road, so it will likely end up being a case of number fudging to enhance the perceived value of a product.