Couldn't high gear ratio replace need for high torque engines? 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?
 A: Torque is specified at a particular rpm - the two things together are a measure of power. If you have lift a heavy truck up a steep hill, you can use a very low gear to "crawl" up - you get up the hill, but go very slowly. If you have a higher torque engine, you don't need such a low gear and will be able to go up the same hill faster. Because the engine develops more power at a given rpm.
A: Consider two engines making the same amount of power, one through displacement (blue) and one through engine speed (red).

If geared appropriately with similar shift points then you will notice that high rpm engine has significantly less ability to accelerate when not near peak rpm. We call this engine elasticity. The ability to accelerate with a similar force pretty much across all rpm range makes cars desirable to drive. The red curve would be a car that is going to be very frustrating as constant shifting is going to be needed to keep the rpms high.
The main difference comes from the way internal combustion engines make torque. There are compromises that need to be made to make high specific power (power at high rpm) that significantly reduce torque at lower rpm. The workaround this problem is either switching cam technology (ex. Honda VTEC) or turbocharging (ex. Audi).  
In the end you want whatever design will give you the widest torque curve for the power requirement. This just makes cars more usable.
