一輛車盛載的重量愈大,那它用的燃油是否愈多?

This is very obvious based on common experience.
The physics behind this fact is:
1. During acceleration, the force required F is the product of mass of the truck m and its accelerationa a, i.e. F = m.a (Newton's second lasw)
The force developed by the engine Fe = F+Ff, where Ff is the frictional force between the road and tyres of the truck. Also, Ff = u.R = u.mg
[where u is the coefficient of friction between road and tyres, and R is the normal reaction of the truck, g is the acceleration due to gravity]
Hence, Fe = ma + Ff = ma + u.mg = m(a+ug)
that is, Fe is proportional to mass of the truck
2. During travelling with a constant speed v, the force developed by the engine is used to overcome the road friction, i.e. Fe = Ff
The power consumed by the engine P = Fe.v = Ff.v = u.mg.v = m(ugv)
Again, Fe, and hence power, is proportional to the mass of the truck.
If it is assumed that the larger the force exerted by the engine, more fuel is to be consumed, it is then pretty clear that a heavily loaded truck would consume more fuel.