Load vs Fuel Efficiency

Google "Conservation of Energy".

"fuel consumption increase when heavy load is towed by tractor as compare to no load what does it actually mean. ??"

"...at heavy load the only difference could be the throttle pedal position which would be more pressed in case of heavy load due to which more diesel will be consumed".

Seems to me you answered your own question.

It will require more fuel to get the tractor+load up to a given speed than the amount of fuel needed to get the tractor (only) up to that same speed. Once that speed is attained only a small amount of 'additional' fuel will be needed to maintain that speed due to the additional amount of frictional forces on the load, compared to the tractor (only).

If the load was significantly larger in cross-sectional surface area than the tractor, you'd notice a large increase in fuel consumption due to the added 'friction' from wind resistance.

^ Unless you're going up-hill .

I'm going to assume you have a tractor with a throttle lever that's attached to the steering wheel column. And that it has detentes to select engine RPM. And you are trying to understand how the fuel consumption is related to this RPM setting. Understand that the tractor engine has a speed governor that the throttle lever is attached too. The governor uses a fly-ball type mechanism (I'm assuming it's an old tractor design), and it will control the engine speed to be constant under load applied to the draw bar. So the carburetor throttle does not stay constant with operator throttle setting. As load increases the throttle opens and burns more fuel.

Same throttle position, same fuel consumption.

You can't have same throttle position at no load and full load.

You can have a constant speed with the same throttle position if the throttle doesn't control fuel flow or air flow, but engine speed. Throttle needs definition here. My lawn mower has no throttle, just a governor. Constant speed under load, pretty much. Of course there has to be a little lag for there to be governor feedback to the carburetor butterfly valve in my case.

Remember the governor.

No the fuel consumption will not be the same.

The prime mover (tractor engine) must supply enough horsepower to propel the tractor and any load attached to it.

The heavier the load being propelled, the more horsepower required hence the lighter the load, the less horsepower required for machine movement.

The more horsepower required; the higher amount of fuel required per second to convert the heat (calories) energy contained in the fuel into rotational movement of the crankshaft.

this is a question that needed to be tested on actual condition.

Loaded theoretically consumes more fuel, rpm will compensate less with load on it.

It's the acceleration part that uses the most fuel...the added mass requires more energy to accelerate from a stopped position....the same with stopping, the kinetic energy is transformed to heat(that's why the brakes get so hot) to stop forward movement....After the truck is at speed the forces at play are wind resistance, friction, and gravity...

only drive downhill for better fuel economy

But, what happens when I reach the South Pole? Will my fuel mileage decrease as I head North?

yes

You get nothing for nothing and very little for a sixpence.

As already said; Even if the throttle lever stays in one place, the diesel pump governor will supply more fuel under heavy load to try to keep the engine speed as set by the throttle lever with no load.