Horsepower is a very real concept, but it is one that most people have absolutely no understanding of when they talk about their “300 HP” cars. Think about this scenario. A hypothetical car will go 25 mph at 2000 RPM in a certain gear. In a lower gear, it will go 25 MPH at 4000 RPM. Well, you think, the engine, because of the torque multiplication of the shorter gear, should supply twice the thrust in the shorter gear, where it is running at 4000 RPM. This, however, is the case only if the engine produces the same amount of torque at 4000 RPM. If the engine is a wheezy thing with a single barrel carburetor that can’t produce much torque above 3000 RPM, shifting to the lower gear will not do any good. This is why modern car engines have all them extra cams and valves, so they can run the needle off the tach while producing very near their maximum torque. Most cars will accelerate very quickly in first gear. If the engine produces near maximum torque all the way to redline, you can maintain this acceleration in first to 35-40 MPH instead of crapping out at 15 MPH.
A Z-06 Corvette engine that can make 505 HP at 6300 RPM is great for making a 3200 lb. car go fast. At 2500 RPM, it may only make about 200 or 225 HP, but 200 easily accessible HP will shove a car along awful fast. A Cat C-15 in an OTR tractor-trailor might also make 500 HP, but that HP comes at around 1800 RPM, and a greater portion of it is available throughout the usable rev range, so it is better suited to dragging 80,000 lbs. all over the country 11 hours a day. Besides, a Cat C-15 engine weighs as much as a Corvette, so it wouldn’t really fit in the car.