Cumulative Model

Prompted by previous posts here and here and here.

A simplified description:

The load on the engine is about same regardless of which gear you are in, the RPMs being lower than peak torque doesn't mean much for fuel consumption. A lot more fuel is injected into the cylinders to provide the power needed to keep the car going at the lower engine speed, and power transmission (for acceleration) is less efficient at the lower engine speed.

Think of your car's mechanics as a set of springs that transfer all the forces acting on the car to the crankshaft. In order to provide the same power in less cycles per minute, more power must be provided by each piston in each cycle (more fuel, bigger explosion). But at slower engine speeds, not all of the blast energy is transferred to the crankshaft. The piston doesn't move as much during the blast (the blast occurs in a smaller space) so more of the energy becomes heat (some of the blast also becomes strain on the piston rod and cylinder). At about 3500 rpm, the pistons are moving at a speed that allows the explosive force to best transfer to the piston (the explosion is pushing as much as possible on the piston during the entire stroke). After 3500 RPM, the piston is moving so fast that not all of the explosive force acts on the piston.

It's also a lot like riding a bicycle. At high gears and low speed, you might not even be able to push hard enough to get going. You mostly just put a lot of strain on the chain. Instead, you use a lower gear and pedal less hard at a more comfortable, medium pace. Once you get going faster, you'll notice that you're spinning the pedals faster, but there is almost no resistance at all and you're wasting a lot of energy moving your feet so fast. So you upshift, resistance increases slightly, your pedaling slows slightly, and you're a lot more comfortable.