An RC car's transmission ratio (aka primary ratio, or internal ratio) is the ratio of how many times the wheels of the car turn for each turn of the spur gear. This ratio is determined by the gears in the transmission and for most cars this ratio is not changeable, so you don't usually need to worry about it. However, if you are trying to determine the final drive ratio of an RC car, you will need to know what the transmission ratio is.
For many cars, you can find the transmission ratio in the manual, but not always. If it isn't there, you can look for the size of the gear on the differential (or axle, and often called a ring gear or crown gear) and divide it by the size of the gear that drives it (which is called a "pinion" usually, but is not to be confused with the pinion gear attached to the motor.)
The differential/axle gear may be driven by a belt or by a direct gear-to-gear contact - either way, you can divide the gear on the differential/axle by the number of teeth on the gear that drives it via belt/gear. This gear is usually called a pinion, and we'll cal it an internal pinion for clarity:
transmission ratio = differential gear teeth / internal pinion teeth
For cars that have multiple gears within their transmission (like 2WD off-road or many drift cars), it's easier just to look up the ratio here on So Dialed or in the manual.
Now that you have the internal ratio, you can calculate the final drive ratio (FDR).
To find your car's internal ratio:
- If you're using our RC Setup App, the internal ratio is probably already provided in the Drivetrain section of your setups
- Our RC Gear Ratio Calculator has many internal ratios listed for popular cars
- Our car database also shows internal ratios at the bottom the page
- The car's manual
Important: cars with just a pinion and spur and no transmission have no internal ratio! If you are doing calculations, you can treat their internal ratio as "1." These are typically on-road "pan" cars such as 1/12th scale pan, 1/10 scale pan, and F1.