求翻译：3）教练车半挂车的使用汽车拖带半挂教练车后，虽然汽车总的燃油消耗量增加了，但分摊到每吨货物上的油耗下降了。拖带挂车后节省燃油的原因有二个：一是带挂车后阻力增加，发动机负荷率增加，使燃油消耗率下降；另一原因是汽车列车的质量利用系数（即装载质量与整车整备质量之比）增大。是什么意思？

3) coach car semi-trailer towing vehicle semi-trailer coach car, although total vehicle fuel consumption has increased, but assessed to a decline in fuel consumption per ton of goods. Towing trailer after saving fuel for two reasons: first trailer resistance increase, increase in engine load rate, bring down the rate of fuel consumption; another reason is the quality of vehicle utilization factor (that is, loading ratio between quality and vehicle curb weight) increases.