Gear Ratio & Speed Calculator
Calculate vehicle speed from gear ratios, RPM, and tire size. Enter values for instant results with step-by-step formulas.
Formula
Speed (MPH) = (RPM × Tire Diameter) / (Gear Ratio × Axle Ratio × 336)
The formula derives from converting RPM (Rotations Per Minute) to Wheel RPM using the gear reduction, then converting Wheel RPM to Miles Per Hour using the tire circumference. The constant 336 consolidates units (inches, minutes, miles).
Worked Examples
Example 1: Highway Cruise
Problem:3000 RPM, 4th Gear (1.00), 3.73 Final, 26" Tire.
Solution:(3000 * 26) / (1.0 * 3.73 * 336) = 78000 / 1253.
Result:Speed: 62.2 MPH.
Frequently Asked Questions
How do I find my gear ratios?
Check your car's owner manual or search online for '[Car Model] transmission ratios'. Common codes: T56, 4L60E, ZF8.
How does tire size affect speed?
Larger tires cover more ground per rotation. They increase top speed (theoretically) but decrease acceleration.
What is a 1:1 gear?
Usually 4th or 5th gear. Input shaft speed equals Output shaft speed. Most efficient gear (no reduction loss).
Why do dynos use 1:1 gear?
To get the most accurate horsepower reading by minimizing transmission friction losses.
What is 'Crawling Ratio'?
Used in off-roading. First Gear × Low Range × Axle Ratio. Can be 100:1 or more.
Background & Theory
Gear Ratios Explained
- Short Gears (High Number, e.g., 4.10): Fast acceleration, low top speed, high RPM. Good for drag racing or towing.
- Tall Gears (Low Number, e.g., 2.73): Slow acceleration, high top speed, low RPM. Good for fuel economy.
Final Drive / Axle Ratio
This is the ratio in the differential. It multiplies every gear in the transmission. Changing the ring and pinion gear (e.g., from 3.08 to 4.10) is a popular "poor man's supercharger" modification to feel faster.
Tire Size Impact
Putting bigger tires on a truck acts like a "taller" gear. It reduces effective torque and makes the speedometer read slower than actual speed. Re-gearing is often needed after lifting a truck.
History
The Mechanical Advantage
Gears provide mechanical advantage. Since the invention of the automobile, engineers have balanced "Torque" (acceleration) with "Speed" (highway cruising). Early cars had 2 or 3 speeds. Modern cars have 8, 9, or 10 speeds to keep the engine in its optimal efficiency range.
Overdrive
In the 1970s fuel crisis, "Overdrive" (gears with a ratio < 1.00) became standard to lower RPM at highway speeds, saving gas. Before that, a 1:1 final gear was common, leading to noisy, thirsty highway driving.
CVTs and EVs
Continuously Variable Transmissions (CVTs) technically have infinite ratios. Electric Vehicles (EVs) often have a single gear because electric motors produce torque instantly across a wide RPM band, eliminating the need for shifting.