Pinion & Spur Gear Ratio Calculator Guide

Understanding how to calculate and adjust your gear ratio lets you tune speed vs. torque without buying a single new part — just swapping gears you may already have.

The Formula

Overall Gear Ratio = (Spur Teeth ÷ Pinion Teeth) × Internal (Transmission) Gear Ratio

Most cars have an internal transmission ratio built into the gearbox/diff setup (often listed in the manual or on the manufacturer's site) — this stays fixed unless you change internal gears, while pinion/spur swaps are the easy, common way to adjust ratio.

Worked Example

Say your car has:

• 87T spur gear
• 19T pinion gear
• Internal ratio of 2.6:1 (from the transmission)

Spur ÷ Pinion = 87 ÷ 19 = 4.58

Overall ratio = 4.58 × 2.6 = 11.9:1

A higher overall ratio number means more torque, less top speed. A lower number means more top speed, less torque.

How Changing Pinion Size Affects Ratio

Since pinion is in the denominator, a smaller pinion increases the overall ratio (more torque, less speed, more motor heat per unit of work), while a larger pinion decreases the ratio (more speed, less torque, generally cooler running per unit of work — until you're pushing the motor's limits).

Swapping just 1–2 teeth on the pinion makes a noticeable difference — this is the most common, cheapest way to fine-tune a car's feel.

How Changing Spur Size Affects Ratio

A larger spur increases the overall ratio (similar effect to a smaller pinion — more torque). A smaller spur decreases it (similar to a larger pinion — more speed). Spur swaps are less common for quick tuning since they typically require more disassembly than a pinion swap.

Estimating Top Speed (Rough Method)

Top Speed ≈ (Motor KV × Battery Voltage) ÷ Overall Gear Ratio × Tire Diameter Factor

This is a simplified estimate — real-world top speed also depends on ESC timing, load, tire size, and surface — but it's useful for comparing how a gearing change should shift your speed relative to your current setup.

Practical Tuning Approach

1. Start at or near the manufacturer's recommended gearing for your motor.
2. Run a pack, then check motor temperature (see our LiPo and ESC guides for safe heat ranges).
3. If cool, try lowering the ratio slightly (bigger pinion) for more speed; if hot, raise the ratio (smaller pinion) for less strain.
4. Adjust one tooth at a time and re-test — gearing changes compound quickly.

Quick Reference

Want	Change
More top speed	Larger pinion or smaller spur
More torque/acceleration	Smaller pinion or larger spur
Less motor heat	Smaller pinion (lower ratio strain)

Gearing math looks intimidating at first, but once you've calculated your car's baseline ratio once, tuning becomes simple tooth-by-tooth adjustment from there.