Gear and sprocket ratio chart
Ratio depends only on tooth count, so this table serves gears, sprockets and timing pulleys alike. Read across for the ratio a given pair produces.
| Drive teeth | Driven 24 | Driven 30 | Driven 36 | Driven 42 | Driven 48 | Driven 60 |
|---|---|---|---|---|---|---|
| 10 | 2.40 | 3.00 | 3.60 | 4.20 | 4.80 | 6.00 |
| 12 | 2.00 | 2.50 | 3.00 | 3.50 | 4.00 | 5.00 |
| 14 | 1.71 | 2.14 | 2.57 | 3.00 | 3.43 | 4.29 |
| 15 | 1.60 | 2.00 | 2.40 | 2.80 | 3.20 | 4.00 |
| 16 | 1.50 | 1.88 | 2.25 | 2.63 | 3.00 | 3.75 |
| 18 | 1.33 | 1.67 | 2.00 | 2.33 | 2.67 | 3.33 |
| 20 | 1.20 | 1.50 | 1.80 | 2.10 | 2.40 | 3.00 |
What the ratio does to speed and torque
Output speed is input speed divided by the ratio. Output torque is input torque multiplied by the ratio, less a few percent for friction. Power is unchanged, because power is speed times torque and one goes up as the other comes down.
A higher numerical ratio means more torque and less speed at the output. On a vehicle that means quicker acceleration and a lower top speed; on a conveyor it means more pulling force at a slower belt.
Final drive and overall ratio
Multiple stages multiply. Overall ratio equals gearbox ratio times final drive ratio times any intermediate reduction. A vehicle in a 3.5:1 first gear with a 4.1:1 differential has an overall ratio of 14.35:1 in that gear.
To find the wheel speed, divide engine rpm by the overall ratio, then convert wheel rpm to road speed from the tyre circumference. To choose a differential ratio for a target cruising rpm, work it backwards from the tyre size and top gear ratio.
Choosing sprocket sizes
On a chain drive, keep the small sprocket to at least 17 teeth where speed and load allow. Below that the chain articulates through too large an angle on each tooth, which increases wear and makes the drive rough. Under 12 teeth is for slow, light duty only.
Avoid ratios that are whole numbers where you can. With a 2:1 ratio the same chain link meets the same tooth every two turns and wear concentrates. A 2.1:1 or 1.9:1 spreads it around, which is why odd tooth counts are common on driven sprockets.
