RC Car Gear Ratio ガイド for Speed vs Torque

Gear ratio is one of the most accessible tuning adjustments on an RC car, and it has a direct effect on how your car accelerates, its top speed, and how hot your motor and ESC run. Changing a pinion or spur gear takes five minutes with basic tools, and the difference in driving feel can be dramatic.

What Is Gear Ratio?

The gear ratio is the relationship between the spur gear (the large gear on the transmission) and the pinion gear (the small gear on the motor shaft). It is calculated by dividing the number of spur gear teeth by the number of pinion gear teeth.

For example, a 90-tooth spur gear with a 20-tooth pinion gives you a ratio of 4.5:1. The motor turns 4.5 times for every one rotation of the spur gear. A 90-tooth spur with a 30-tooth pinion gives you 3:1, meaning the motor turns only 3 times per spur rotation.

Higher Ratio vs Lower Ratio

A higher ratio (smaller pinion or larger spur) means more torque and less top speed. The motor does not work as hard to turn the wheels, which means lower temperatures and longer run times. The car accelerates harder out of corners but tops out at a lower speed.

A lower ratio (larger pinion or smaller spur) means less torque and more top speed. The motor works harder, generating more heat. The car is faster on straightaways but slower to accelerate and may struggle on rough terrain or steep inclines.

How to Choose

Your ideal gear ratio depends on where and how you drive:

• Technical tracks with tight corners: higher ratio for better acceleration and throttle response
• Fast tracks with long straightaways: lower ratio to maximize top speed
• Off-road bashing: higher ratio for torque to handle rough terrain, jumps, and landings
• Drag racing: lowest ratio your motor and battery can handle for maximum speed

Motor Temperature Is Your Guide

The most practical way to dial in your gear ratio is by monitoring motor temperature. After a 5 to 10 minute run, check your motor temperature with an infrared thermometer:

• Under 150F (65C): you have room to gear down (lower ratio) for more speed
• 150-170F (65-77C): ideal operating range for most brushless motors
• Over 170F (77C): gear up (higher ratio) to reduce load and heat

Running a motor too hot shortens its lifespan and can damage the ESC. Always check temperatures when making gearing changes, especially in warm weather.

Common Mistakes

• Gearing too low (too aggressive) for the available traction. If your wheels spin constantly, you are wasting energy as heat instead of speed
• Ignoring ambient temperature. A gear ratio that works fine in spring may overheat the motor in summer
• Not checking gear mesh after changing gears. The pinion and spur need a thin paper gap between them for smooth operation

Quick Reference

If you are unsure where to start, use the stock gear ratio your manufacturer recommends as a baseline. Make one-tooth pinion changes at a time, test, and check temperatures. Small changes add up, and a methodical approach prevents overheating and damaged equipment.

Gearing is one of those RC fundamentals that seems intimidating until you try it. A couple of gear changes and temperature checks will teach you more about how your car works than reading a dozen forum posts. Get some extra pinion gears in a few sizes and start experimenting.