Gear Ratio & Speed Calculator
Calculate vehicle speed from RPM, gear ratios, final drive, and tire size. Optimize drivetrain setups for acceleration or top speed.
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 in Direct Drive
Problem:3,000 RPM in 4th gear (1.00:1 direct drive), a 3.73 final drive and 26-inch tires. How fast is the vehicle actually travelling?
Solution:Speed (MPH) = (RPM x Tire Diameter) / (Gear Ratio x Axle Ratio x 336) Numerator: 3,000 x 26 = 78,000 Denominator: 1.00 x 3.73 x 336 = 1,253.28 Speed = 78,000 / 1,253.28 = 62.2 MPH Running it backwards to find cruise RPM at 70 MPH in a 0.72 overdrive: RPM = (Speed x Gear x Axle x 336) / Tire = (70 x 0.72 x 3.73 x 336) / 26 = 63,165 / 26 = 2,429 RPM That 570 RPM drop between direct drive and overdrive at highway speed is the entire reason overdrive gears exist - less friction, less noise, better fuel economy.
Result:62.2 MPH at 3,000 RPM in direct drive; the same drivetrain cruises 70 MPH at 2,429 RPM in a 0.72 overdrive.
Example 2: First-Gear Redline - Where Does Second Gear Have To Start?
Problem:Same 3.73 axle and 26-inch tires. First gear is 3.06:1 and the engine redlines at 6,000 RPM. What is the maximum speed in first?
Solution:Denominator: 3.06 x 3.73 x 336 = 11.4138 x 336 = 3,835.0 Numerator: 6,000 x 26 = 156,000 Speed = 156,000 / 3,835.0 = 40.7 MPH Why this number matters for gear selection: it tells you the shift point. If second gear is 1.78:1, the engine speed immediately after the 1-2 shift is: RPM = (40.7 x 1.78 x 3.73 x 336) / 26 = 3,491 RPM That 2,509 RPM drop (6,000 down to 3,491) is the ratio split. A wide split like this drops the engine well below its torque peak and feels sluggish; close-ratio gearboxes exist to keep that drop nearer 1,500 RPM so the engine never leaves its power band.
Result:40.7 MPH at redline in first; the 1-2 shift drops the engine to 3,491 RPM, a 2,509 RPM split.
Example 3: Oversized Tires - Speedometer Error and Re-Gearing
Problem:A truck built around 26-inch tires is fitted with 33-inch tires. At 3,000 RPM in direct drive with the 3.73 axle, what is the real speed, and what axle ratio restores the original gearing?
Solution:True speed on 33-inch tires: (3,000 x 33) / (1.00 x 3.73 x 336) = 99,000 / 1,253.28 = 79.0 MPH But the speedometer is still calibrated for 26-inch tires, so it reads 62.2 MPH. The error: 33 / 26 = 1.2692, so the speedometer under-reads by 26.9%. A shown 62 MPH is a real 79 MPH - which is how oversized tires produce speeding tickets and mileage that quietly undercounts. Restoring the original gearing: Required axle ratio = 3.73 x (33 / 26) = 3.73 x 1.2692 = 4.73 There is no 4.73 ring-and-pinion, so choose from stock ratios: At 4.88: 99,000 / (4.88 x 336) = 99,000 / 1,639.68 = 60.4 MPH - slightly shorter than stock, better for towing. At 4.56: 99,000 / (4.56 x 336) = 99,000 / 1,532.16 = 64.6 MPH - slightly taller, better for highway economy.
Result:79.0 MPH true against a 62.2 MPH indicated reading - a 26.9% under-read. A 4.88 or 4.56 axle brings the drivetrain back to stock.
Frequently Asked Questions
What is the constant 336?
It combines conversion factors: 63360 inches/mile divided by 60 minutes/hour and Pi. (Exact is closer to 336.13).
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.
What is 'Final Drive'?
The gear reduction in the differential (Rear Axle). It affects all gears equally.
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.
What is RPM drop?
The drop in RPM when shifting up. Close-ratio gearboxes have smaller drops to keep the engine in the power band.
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.