What is the gear ratio formula?
Ratio = Driven Teeth / Driver Teeth (or equivalently, Driver RPM / Driven RPM). For example, a 20T driver meshing with a 60T driven gives a 60/20 = 3:1 ratio. The output turns three times slower than the input but with three times the torque. The same formula applies to gear diameters in mating gears (since diameter is proportional to teeth).
Why is tooth count used instead of diameter?
For gears to mesh, they must have the same "module" (metric) or "diametral pitch" (imperial) — the tooth size. Diameter alone doesn't guarantee they'll mesh; tooth count does. Two gears with matching pitch and 20/40 teeth will always mate correctly. Diameter is determined by tooth count and pitch: D = T × module / π (or T / pitch in inches).
Do gears reverse direction?
External gears (the common type) reverse direction when they mesh — that's why a gear pair has the output spinning the opposite way from the input. Internal (ring) gears maintain direction. Each gear pair adds another reversal: 1 pair = reversed, 2 pairs = same direction, 3 pairs = reversed, etc. An "idler gear" (intermediate gear) only changes direction, not the overall ratio.
How do I calculate vehicle speed from RPM?
MPH = (Engine RPM × Tire Diameter in inches) / (Trans Ratio × Final Drive × 336.13). The constant 336.13 comes from unit conversions (inches/min → MPH, with π for circumference). Wheel RPM = Engine RPM / (Trans × Final Drive), then multiply by tire circumference and convert to your speed unit. Tire diameter is the critical measurement — a 1-inch larger tire gives about 4% higher speed at the same RPM.
What's a "final drive" ratio?
The final drive (or differential ratio, axle ratio) is the fixed reduction between the transmission output and the wheels. It's applied IN ADDITION to whatever transmission gear is engaged. A typical 3.73 final drive means the wheels turn once for every 3.73 turns of the driveshaft. A higher number gives quicker acceleration but lower top speed; lower number gives the opposite. Final drive is fixed for the vehicle's life unless physically changed.
What is overdrive?
A transmission gear ratio LESS than 1:1 — typically 0.6-0.85. The output shaft spins FASTER than the input. Used for highway cruising at low engine RPM for fuel economy and reduced noise. 4th gear is often "direct" (1:1), so 5th and 6th are overdrive. Modern automatics have 8-10 speeds with multiple overdrive ratios. Disengage overdrive for towing or hill climbing where extra torque is needed.
How do larger tires affect speedometer accuracy?
Larger tires cover more distance per revolution. If you install tires 5% larger in diameter, your actual speed is 5% higher than the speedometer reads. Example: speedo says 60 MPH, you're actually doing 63 MPH. This affects odometer too (under-reports miles). Many off-roaders need ECU recalibration after lifting and installing taller tires to correct speedometer/transmission shift points.
Are gears more efficient than belts or chains?
Spur and helical gears: 95-99% efficiency (very high). Belt drives: 85-95% (some slip). Chain drives: 95-98%. Worm gears: 50-90% (lowest, due to sliding). Gears are generally most efficient but cost more, require precise alignment, and need lubrication. Belts handle misalignment and shock loads better but slip. Chains are great for high-torque parallel shafts. Each has its place.
What's a compound gear train?
Multiple gear pairs in series — the output of one pair drives the input of the next. Overall ratio = product of individual ratios. A 4:1 followed by 3:1 gives 12:1 reduction. Compound trains achieve high ratios in compact space and allow multiple speeds. Modern automatic transmissions, gearboxes, and clocks all use compound gear trains.
Why are some gears made of plastic?
Plastic gears (typically acetal/Delrin or nylon) are used where load is low, noise must be minimized, and self-lubrication is helpful. Found in printers, kitchen appliances, toys, and instrument drives. Plastic-on-metal pairs reduce wear and noise vs metal-on-metal. Drawbacks: lower load capacity, lower temperature tolerance, can creep under sustained load. Steel and bronze are used for high-load industrial gears.
How do I know my car's gear ratios?
Check the owner's manual or service manual — usually in specifications. Online forums for your specific vehicle model often list common ratios. The final drive can be found on the differential tag (a metal plate on the housing). For aftermarket transmissions or rebuilt engines, ratios may be marked on the case. Apps like Torque Pro (with OBD-II) can sometimes log gear engagement.
Why do gears need to be the same module/pitch?
Module (or pitch) defines tooth size. For gears to mesh, teeth must be the same size — like puzzle pieces. A 1-module gear has teeth roughly 3.14mm tall; a 2-module gear has teeth twice as big. Mixing modules causes interference (gears bind) or improper engagement (gears strip). When designing or replacing gears, the module/pitch must match all meshing partners. Common modules: 0.5, 1, 1.5, 2, 2.5, 3 (metric); 16, 20, 24, 32 DP (imperial).
