So weight plays no/ not so considerable role here?
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Of course everyone knows this law, I'm not trying to act like anything here, but why did you completely ignored the huge diff. in weight?? While there is considerable amount of difference in bhp/ton numbers of these two cars.
A car with more weight will naturally require more power just to move its parts,
Well, this will be way too technical for me to handle...never was good in physics anyways.
However, I do understand some basics. Hauling/towing does not mean you require acceleration. A larger displacement engine is the prime requirement (this is what I have been made to understand by my cousin who works with an Indian auto company and these have been my observations of all towing vehicles I have come across the globe) for hauling. Those cars are not meant to be driven at speeds or are not intended to accelerate fast.
Of course, the weight of the vehicle itself matters, but not so much as you may think.
There are a lot of engines which are less than 1.6 litres in displacement, but are a lot more powerful and efficient than their bigger siblings. These engines are never fitted in vehicles which are intended to be hauler's or which are to be used as tow vehicles. This is just a general rule, and case to case scenarios will definitely vary.
These hauler's or towing vehicles come into their own when you have to carry huge loads on ghats or treacherous roads. You may say that the ertiga is also used by people in himalayan areas in India, and they are doing well.. I will and do agree with you. However, the car has to be worked upon a lot on these roads. The innova/tavera/xylo/sumo type cars are seriously easier to drive in these conditions. I was in ladakh 2 years back and I remember a local dealer from tata had brought the then new aria to the taxi cab operators for demo. The first question of all the operators was the displacement. Many just rejected the car based solely on the displacement. Of course, some were impressed and did end up booking one...it was fun watching the scenario as a tourist.
As I said, I will not be able to explain this in technical terms myself, since I am not so technically inclined; however, this is what I have been made to understand and this also matches my observations. Exceptions are always going to be a rule.
You will not miss the displacement as long you drive on plains and on good roads.
My cousin did mention that things will definitely change in the future and globally manufacturers are working on tech which will make the bigger displacement engines obsolete, but it still a long way.
So mass of the vehicle is a factor, but not as important as you may think.