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Miata Gearing Calculator

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Have you ever wondered how the speed of a car is calculated based on its engine RPM, gear ratio, differential ratio, and tire diameter? Well, that’s where a vehicle speed calculator comes into play. In this article, we’ll delve into the workings of this calculator, its formula, and provide step-by-step examples to help you understand its functionality better.

What is a Vehicle Speed Calculator?

A vehicle speed calculator is a tool used to estimate the speed of a vehicle based on various factors such as engine RPM, gear ratio, differential ratio, and tire diameter. It helps enthusiasts, mechanics, and engineers analyze the performance characteristics of a vehicle and make informed decisions about gear ratios, tire sizes, and other related aspects.

How Does it Work?

The calculator operates on a simple yet powerful formula:

Vehicle Speed (mph or km/h) = (RPM * Tire Circumference * Gear Ratio * Differential Ratio) / Constant

Let’s break down each component:

  • Engine RPM: This refers to the engine speed in revolutions per minute.
  • Gear Ratio: It’s the ratio of the number of revolutions of the transmission output shaft to one revolution of the engine’s crankshaft.
  • Differential Ratio: This ratio determines how many revolutions of the driveshaft correspond to one revolution of the wheels.
  • Tire Diameter: The overall diameter of the tires, which influences how far the vehicle travels with each wheel revolution.

The constant in the formula depends on the units of measurement:

  • For miles per hour (mph), the constant is 336.13 if the tire circumference is in inches.
  • For kilometers per hour (km/h), the constant is 168.622 if the tire circumference is in meters.


A Miata Gearing Calculator helps you understand how the gears in a Mazda Miata affect the car’s performance, especially in terms of speed and RPM (revolutions per minute). The formula to calculate the effect of gearing involves several key components: the gear ratios, the final drive ratio, tire size, and engine speed. Here’s a simplified version of the formula:

Speed = (RPM × Tire Circumference) / (Gear Ratio × Final Drive Ratio × Conversion Factor)

Here’s a breakdown of each component in simple terms:

  1. RPM (Revolutions Per Minute): This is how fast the engine is spinning. It’s the “revolutions per minute” of the engine’s crankshaft.
  2. Tire Circumference: This is the distance around your tire. It’s important because it determines how far your car travels with each revolution of the tire. The circumference can be calculated based on the tire size.
  3. Gear Ratio: Each gear in your transmission has a ratio, which determines how many times the engine’s output shaft spins for each revolution of the transmission’s input shaft. Lower gears (like 1st and 2nd) have higher ratios for more power at low speeds, while higher gears (like 5th and 6th) have lower ratios for better fuel efficiency at high speeds.
  4. Final Drive Ratio: This is the ratio in your car’s differential, which is part of the drivetrain. It further reduces the speed from the transmission before it gets to the wheels, providing more torque.
  5. Conversion Factor: Since the formula involves different units of measurement, a conversion factor is used to convert the result into a usable unit, like miles per hour (mph) or kilometers per hour (km/h).

Step-by-Step Example

Let’s walk through an example calculation to understand how the vehicle speed is determined:

Suppose we want to find the speed of a car at 3000 RPM in 3rd gear, with the following specifications:

  • Gear Ratio (3rd Gear): 1.300
  • Differential Ratio: 4.100
  • Tire Diameter: 23 inches
  1. Calculate Tire Circumference: Circumference = Diameter * π ≈ 23 * π ≈ 72.26 inches
  2. Plug the values into the formula: (mph) = (3000 * 72.26 * 1.300 * 4.100) / 336.13 ≈ (1.300 * 4.100 * 336.13) / 3000 * 72.26 ≈ 121.0 mph

Thus, the estimated speed of the car at 3000 RPM in 3rd gear would be approximately 121.0 mph.


In conclusion, the vehicle speed calculator is a valuable tool for understanding the relationship between engine RPM, gear ratio, differential ratio, tire diameter, and vehicle speed. It provides enthusiasts and professionals with insights into a vehicle’s performance characteristics and helps optimize various parameters for enhanced performance. Whether you’re a car enthusiast or an automotive engineer, this calculator can be a handy tool for making informed decisions and improving vehicle performance.

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