Engines · Drivetrain · Demo
The engine only works between 1 000 and 7 000 rpm — gears translate that narrow band into every speed a car needs.
A = always the strongest gear for the current speed, like an automatic kicking down.
The final drive multiplies every gear at once: shorter (higher number) for punch, taller for relaxed cruising and top speed. Watch the whole sawtooth slide when you move it.
Real mechanics, one honest simplification: full throttle everywhere, and a fixed 90% driveline efficiency. Force at the wheels is engine torque × gear × final drive ÷ wheel radius; drag is v² aerodynamics plus rolling resistance. The sawtooth and the drag curve between them decide everything a road car can do.
In plain terms
An engine is fussy about speed: below about 1 000 rpm it stalls, past 7 000 it destroys itself, and it only pulls really hard in the middle. The wheels, meanwhile, need everything from a standstill shove to a 200 km/h cruise. A gearbox is the translator — each gear trades speed for force like a lever, multiplying the engine’s twist in first gear and stretching its legs in fifth. Same engine, five personalities.
When a small gear drives a big one, the big one turns slower but harder — the trade is exact, like a crowbar. A 3.6:1 first gear multiplied by a 3.9:1 final drive hands the wheels fourteen times the engine’s torque. That, not engine power, is why a modest hatchback can spin its tyres from rest.
The gearbox never adds power — whatever kilowatts go in come out, minus a little friction. What it changes is the mix: fewer revolutions, more twist per revolution. This is why the tractive-force curves of all five gears kiss the same invisible ceiling — the engine’s power curve, translated to the road.
Plot wheel force against road speed and a geared car draws a falling sawtooth: each gear starts strong and fades as revs run out, then the next gear catches the baton lower down. The car accelerates on whatever force is left above the drag curve — and where the last tooth finally sinks into the drag, that is top speed.