Transmission: provides selectable gear ratios between the engine's crankshaft and the drive wheels. Without a transmission, engines (which need to spin in a specific RPM range — typically 600-7000 RPM) couldn't efficiently propel a car across all speeds (0-80+ mph). Lower gears multiply torque (more force, less speed) for starting from rest, climbing hills, accelerating; higher gears allow cruising at low RPM (better fuel economy, less noise). MANUAL TRANSMISSION (stick shift) parts: (1) CLUTCH — friction disk between engine flywheel and transmission input shaft; engaged/disengaged by clutch pedal; (2) GEARS — sets of meshed gears in different ratios (1st, 2nd, 3rd, 4th, 5th, 6th, plus reverse); shift fork engages chosen pair; (3) SYNCHRONIZERS — match speeds between gears before engagement to prevent grinding; (4) SHIFT MECHANISM — linkage from gear selector lever to transmission; (5) DIFFERENTIAL — splits torque between drive wheels, allowing them to turn at different speeds in turns. Typical gear ratios: 1st = 3.5-4.5 (high torque multiplication); 2nd = 2.0-2.5; 3rd = 1.5; 4th = 1.0 (1:1, direct drive); 5th = 0.8 (overdrive, reduces engine RPM); 6th-10th = lower for fuel economy. AUTOMATIC TRANSMISSION (most US cars): (1) TORQUE CONVERTER — fluid coupling between engine and transmission; allows engine to keep running with vehicle stopped; some torque multiplication at stall; replaces clutch in conventional automatics; (2) PLANETARY GEAR SETS — multiple ratios via sun-planet-ring gear configurations; selected by friction clutches and bands; (3) HYDRAULIC CONTROL UNIT — fluid pressure controls clutch/band engagement; modern: electronically controlled; (4) TORQUE CONVERTER LOCKUP — locks input-output for direct drive at cruise (eliminates fluid coupling slippage for efficiency); (5) MODES — typically P (park), R (reverse), N (neutral), D (drive, all forward gears), 2 or L (limited gear range). Other transmission types: (1) CVT (Continuously Variable Transmission) — uses belts and variable-diameter pulleys; infinite ratios within range; smooth, efficient; (2) DUAL-CLUTCH (DCT/DSG) — like two manual transmissions; odd and even gears each have own clutch; ultra-fast shifts; sporty; (3) AUTOMATED MANUAL — manual transmission with computer-actuated clutch and shifts; (4) ELECTRIC VEHICLES — typically single-speed (electric motors deliver max torque from 0 RPM); some performance EVs use 2-speed.
Awd/4wd: power to all wheels; AWD typically constant or variable, 4WD typically driver-engaged for off-road.
Differential: allows wheels to rotate at different speeds in turns (outside wheel travels farther); standard 'open' differential sends power to wheel with least traction; limited-slip and locking differentials counteract this for traction.