RC Gear Ratio Calculator
Gear ratio
5.00
How it works
Gear ratio determines the trade-off between torque and speed in RC vehicles. The RC Gear Ratio Calculator computes final drive ratio, top speed, and motor load from pinion, spur gear tooth counts, and tire dimensions.
**Gear ratio fundamentals** Final drive ratio = spur gear teeth divided by pinion gear teeth. A 90-tooth spur with an 18-tooth pinion gives a ratio of 5:1 — the motor spins 5 times for every 1 rotation of the drive shaft. Higher ratio = more torque but lower top speed. Lower ratio = higher top speed but less torque.
**Top speed calculation** Top speed = (motor KV x battery voltage x tire circumference) divided by (final ratio x 60). Example: 3,000 KV motor, 7.4V battery, 6.5-inch tire circumference, 5:1 ratio gives approximately 16 mph (actual speed is lower due to drivetrain losses).
**Pinion selection** Smaller pinion (fewer teeth): more torque, less speed, cooler motor. Larger pinion (more teeth): higher speed, higher current draw, hotter motor. Change pinion size by 1 to 2 teeth at a time and monitor motor temperature.
**Crawlers vs. bashers** Rock crawlers use high gear ratios (10:1 to 30:1) for low-speed torque to climb obstacles. Bashers and race cars use low ratios (3:1 to 6:1) for high speed. Gearing is the primary tuning lever for RC vehicle performance.
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Frequently Asked Questions
- Count every tooth on the physical gear. For pinion gears: small tooth counts (12–25 teeth) are easy to count by hand — count around the circumference and recount once to verify. For spur gears with many teeth (60–100+): divide the gear into quadrants and count one quadrant, multiply by 4, then count the remaining teeth. Alternatively: photograph the gear against a white background, import to a computer, enlarge, and count from the image. Most RC manufacturers list pinion and spur tooth counts in the manual or on the product page for the specific vehicle and gear type. For aftermarket gears, tooth count is always listed in the product specifications.
- Bashing (general rough terrain running) benefits from a moderate ratio: 5:1 to 7:1 with most brushless systems. Higher ratio (7:1+): excellent low-end punch, great for rough terrain, jumps, and obstacles, motor runs cooler, shorter top speed. Lower ratio (4:1 to 5:1): higher top speed on smooth pavement, higher motor temperature, may feel sluggish on rough terrain. For most stadium-style RTR bashers on varied terrain, the stock gear ratio is typically well-chosen. Change gearing when: you're consistently running at full throttle without reaching top speed (go lower ratio), or you're burning up motors and ESCs from sustained current draw (go higher ratio).
- Spur gear stripping usually results from: incorrect mesh (pinion too far from spur, causing rocking contact), running without a spur gear cover that keeps debris out, running gear ratios that put excessive load on the drivetrain, or using mismatched gear module sizes (MOD). For correct mesh: the gears should mesh with minimal backlash — you should be able to rotate by hand with slight resistance but no binding. About 0.5mm backlash is typical. Check mesh after installation: hold the motor mount plate and rock the pinion against the spur — minimal play is correct. Use petroleum jelly or gear grease on the gear mesh — dry gears wear much faster and run hotter.
- Gear module (MOD or module) is the standardized measurement of tooth size: module = pitch diameter ÷ tooth count. Common RC modules: MOD 0.6 for small/micro RCs, MOD 0.8 for 1/16 and some 1/10 scale, MOD 1 for most 1/10 scale, MOD 1.5 for 1/8 scale and larger. Module MUST match between pinion and spur gear — two gears of different module cannot mesh properly. A MOD 1 pinion will not work with a MOD 0.8 spur. When buying replacement pinions: confirm the module matches your spur gear. The tooth profile angle (pressure angle) should also match — most RC gears use 20-degree pressure angle, but some older designs use 14.5 degrees; mixing pressure angles causes noise and rapid wear.