I've noticed that there is a correlation between displacement and torque, so trucks, SUVs etc. have large displacement engines to produce lots of torque to move heavy loads.
Sports cars have lower displacements than most trucks, but produce as much if not more power, but typically much less torque. They also tend to get better gas mileage. This of course has a lot to do with weight, aerodynamics etc, but I can't help but notice these engines seem more efficient.
Regardless, it costs lots of money to develop and maintain different engine lines (in 1954, for instance, ALL Chevy's had the SAME inline 6 ). So there needs to be significant advantages to different types of engines.
Now forget all that supposition about displacement and gas mileage, here is the actual question:
Why don't vehicles with more torque-intensive loads use the same engines as vehicles with more power-intensive loads, just with different transmissions and gear ratios?