Gear Ratio Calculator
Analyze drivetrain gearing, road speed, and engine RPM for automotive, motorsport, and off-road applications. By inputting your transmission gear ratio, differential final drive ratio, tire diameter, and engine operating RPM, this tool computes true vehicle speed and cruising RPM with zero server delay.
Gearbox ratio (e.g., 1.00 for direct 4th/5th, 0.68 for overdrive)
Axle differential ring-and-pinion ratio (e.g., 3.42, 3.73, 4.10)
Total standing tire diameter from ground to tread top
Current or target crankshaft revolutions per minute
Calculated vehicle ground speed in miles per hour
How to Use This Calculator
- Obtain your vehicle transmission gear ratios from your owner manual or specification sheet.
- Enter the specific gear ratio you wish to analyze (e.g., 1st gear for launch, top gear for cruise).
- Input the differential axle ratio (final drive).
- Enter your tire outside diameter (or calculate it using our Tire Size Calculator).
- Set your operating engine RPM to view exact speed in both MPH and km/h.
Formula & Calculation Methodology
Road speed is calculated from engine RPM and tire rolling diameter divided by total driveline reduction. The automotive engineering constant 336.135 translates tire circumference in inches and minutes into miles per hour.
Worked Example
Direct drive 1.00:1 transmission in 4th gear, 3.73 final drive, 26-inch tires at 3000 RPM
Frequently Asked Questions
What is the mechanical difference between a "short" and a "tall" final drive ratio?↓
A "short" (numerically higher) ratio like 4.10:1 multiplies torque more aggressively for faster acceleration but increases engine cruising RPM. A "tall" (numerically lower) ratio like 3.08:1 lowers highway cruising RPM and saves fuel but softens acceleration.
How does changing tire size alter effective gearing?↓
Installing larger diameter tires increases vehicle ground speed per revolution, creating the exact same effect as installing a taller (numerically lower) final drive gear.
What is an "overdrive" gear ratio?↓
An overdrive gear has a ratio less than 1.00:1 (such as 0.70:1 or 0.65:1), meaning the transmission output shaft spins faster than the engine crankshaft, reducing engine RPM at highway cruising speeds.
Does this calculator account for automatic transmission torque converter slip?↓
This calculation models direct mechanical coupling (manual transmissions, dual-clutch gearboxes, or torque converter automatic transmissions in lockup mode). Under heavy acceleration with an unlocked torque converter, engine RPM will read 5% to 15% higher.
What limitation should I keep in mind regarding dynamic tire deformation at high speed?↓
Centrifugal forces expand tire diameter slightly at speeds above 100 mph, marginally altering the rolling circumference compared to static standing measurements.
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Open calculator →Source Citation & Disclaimer
Informational tool only; not licensed tax, legal, or financial advice.