Calculate Vehicle Speed From Rpm

Vehicle Speed Equation:

\[ \text{Speed (mph)} = \left( \frac{\text{RPM}}{\text{Gear Ratio} \times \text{Final Drive Ratio}} \right) \times \left( \frac{\text{Tire Diameter (inches)} \times \pi}{336} \right) \]

RPM (engine rotations per minute):

rpm

Gear Ratio (transmission):

dimensionless

Final Drive Ratio:

dimensionless

Tire Diameter:

inches

Vehicle Speed:

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1. What is the Vehicle Speed Equation?

The vehicle speed equation calculates the speed of a vehicle based on engine RPM, transmission gear ratio, final drive ratio, and tire diameter. This is useful for performance tuning, gear selection, and understanding vehicle dynamics.

2. How Does the Calculator Work?

The calculator uses the vehicle speed equation:

\[ \text{Speed (mph)} = \left( \frac{\text{RPM}}{\text{Gear Ratio} \times \text{Final Drive Ratio}} \right) \times \left( \frac{\text{Tire Diameter (inches)} \times \pi}{336} \right) \]

Where:

\( RPM \) — Engine rotations per minute
\( Gear Ratio \) — Transmission gear ratio
\( Final Drive Ratio \) — Differential gear ratio
\( Tire Diameter \) — Diameter of tire in inches
\( \pi \) — Mathematical constant (~3.14159)
336 — Conversion factor for mph

Explanation: The equation converts engine RPM to wheel RPM using gear ratios, then converts wheel RPM to vehicle speed using tire circumference.

3. Importance of Speed Calculation

Details: Accurate speed calculation helps in gear selection, performance tuning, and verifying speedometer accuracy. It's essential for racing applications and vehicle modifications.

4. Using the Calculator

Tips: Enter RPM (typically 1000-7000 for most vehicles), gear ratio (found in vehicle specs), final drive ratio, and measured tire diameter. All values must be positive numbers.

5. Frequently Asked Questions (FAQ)

Q1: How do I find my gear ratios?
A: Gear ratios can be found in your vehicle's manual or from manufacturer specifications. Each gear (1st, 2nd, etc.) has its own ratio.

Q2: What's a typical final drive ratio?
A: Final drive ratios typically range from 2.73 to 4.10 for passenger vehicles, with higher numbers common in trucks and performance cars.

Q3: How do I measure tire diameter?
A: Measure from ground to top of tire (loaded) or calculate from tire size markings (e.g., 225/45R17 = ((225*0.45*2)/25.4)+17).

Q4: Why does tire diameter affect speed?
A: Larger tires rotate fewer times per mile than smaller tires, effectively changing the final drive ratio.

Q5: Can I use this for km/h instead of mph?
A: Yes, but you would need to change the conversion factor from 336 to ~208.86 (or convert mph result to km/h by multiplying by 1.60934).