RC Pinion/Spur Gear Ratio Calculator
Calculate your external gear ratio, total gear ratio, and rollout from pinion and spur tooth counts. Internal (transmission/diff) ratio is left as an optional field you fill in yourself, since it varies by brand and model and isn’t something we try to track for you.
How this works
- External gear ratio is just spur teeth ÷ pinion teeth — the ratio between your pinion (on the motor) and spur (on the diff/gearbox input shaft).
- Internal ratio is whatever additional reduction happens inside your gearbox/transmission after the spur gear. This is different across brands and even across kits from the same brand, so it’s not built into this tool. Check your manual or spec sheet for this number — it’s usually listed as “internal ratio,” “diff ratio,” or “final drive.” If you leave it at 1, results reflect external ratio only.
- Total gear ratio = external ratio × internal ratio. This is what actually determines your motor RPM to wheel RPM relationship.
- Rollout is how far the vehicle travels per motor revolution — a handy way to compare gearing across setups without needing motor RPM at all.
Varies by brand/model — check your manual. Leave at 1 to see external ratio only.
How it works: External Ratio = Spur ÷ Pinion. Total Ratio = External × Internal. Rollout = (π × Wheel Diameter) ÷ Total Ratio — the distance traveled per motor revolution.
Frequently Asked Questions
What’s the difference between external and internal gear ratio?
External ratio is just spur teeth divided by pinion teeth — the visible gear mesh you can change by swapping pinions or spurs. Internal ratio is an additional reduction built into the gearbox or differential itself, which varies by brand/model and isn’t adjustable by changing pinion/spur.
Why isn’t internal ratio built into this calculator?
Internal ratios differ across brands and even between kits from the same manufacturer, and they change as new models are released. Rather than maintain a chart that’s constantly out of date, we left it as a field you fill in from your own manual — leave it at 1 if you only want the external ratio.
Why doesn’t this calculator estimate top speed?
Most racing motors are rated in turns (like 13.5T or 17.5T) rather than Kv, and turns don’t convert to a fixed Kv — it varies by winding style and brand. Any speed estimate built on a guessed Kv would be misleading, so we left it out rather than give you a number that isn’t trustworthy.
What pinion/spur combo should I run?
It depends on your motor’s turns rating, battery voltage, track/terrain, and how much motor heat you’re willing to manage — smaller pinions (numerically higher ratio) give more low-end punch and less heat, while larger pinions (numerically lower ratio) push top speed higher at the cost of more heat and current draw. Many racers gear up gradually and monitor motor/ESC temperature after each run.
What is “rollout” and why does it matter?
Rollout is how far the vehicle travels for one full motor revolution. It’s a quick way to compare gearing across different setups without needing to know motor RPM — two setups with the same rollout will behave similarly in terms of acceleration and top-end feel, even if their raw ratios look different.