Gear Ratio and Vehicle Top Speed Simulator

Project output speeds based on transmission ratio, final drive axle ratio, engine RPM, and tire diameter. Instant MPH and KM/H readouts with no button clicks required.

Panel 1: Drivetrain Specifications
Set engine RPM and gear ratios. All fields update speed instantly.
5,000
Enter the ratio directly (e.g. 3.82 for 1st gear, 0.64 for 6th overdrive)
Choosing a preset fills the ratio above automatically
Common values: 3.08, 3.31, 3.55, 3.73, 3.91, 4.10, 4.56
Panel 2: Tire Specifications
Choose how you want to specify tire size.
Common examples: 26 in (small), 28 in (mid), 30-33 in (truck)
Panel 3: Telemetry and Speed Output
Top Speed
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Miles Per Hour
Top Speed
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Kilometers Per Hour
Gear Trans Ratio MPH at RPM KM/H at RPM

Gear ratios used: 1st 3.82, 2nd 2.26, 3rd 1.44, 4th 1.00, 5th 0.74, 6th 0.64. Axle ratio and tire size from your current inputs.

Key Terms Explained
Gear Ratio
The ratio of input shaft revolutions to output shaft revolutions in a gearbox. A ratio of 3.82:1 means the engine rotates 3.82 times for every one rotation of the output shaft.
Final Drive (Axle Ratio)
The gear ratio inside the rear or front differential. A ratio of 3.73:1 means the driveshaft turns 3.73 times per single wheel revolution. Higher numbers equal more torque multiplication.
Overdrive
Any transmission gear with a ratio below 1:1 (for example 0.74 or 0.64). In overdrive, the output shaft spins faster than the input, allowing the engine to run at low RPM at highway speed.
Engine RPM
Revolutions Per Minute. The number of times the engine crankshaft completes a full rotation each minute. Street engines typically operate between 600 RPM at idle and 6,000 to 8,000 RPM at redline.
Tire Diameter
The total outer diameter of a mounted tire, measured in inches. The circumference (pi times diameter) determines how far the vehicle moves per wheel revolution. Larger diameter means more distance covered per revolution.
Aspect Ratio
In a metric tire code such as 275/40R20, the "40" is the aspect ratio: the sidewall height expressed as a percentage of the tread width. A 40-series tire has a sidewall 40 percent as tall as it is wide.
Redline
The maximum safe engine RPM, marked in red on the tachometer. The speed calculated at redline in a given gear represents the theoretical top speed for that gear ratio and tire combination.
Aerodynamic Drag
Air resistance that grows exponentially with speed (proportional to velocity squared). At highway speeds, aerodynamic drag is typically the dominant force limiting top speed, which is why real-world top speeds fall short of theoretical calculations.

The Complete Guide to Gear Ratios and Vehicle Top Speed

Whether you are selecting axle gears for a performance build, checking whether a tire size upgrade will push your speedometer reading off, or simply curious about the math behind your car's drivetrain, this simulator gives you accurate, instant answers based on the actual physics of gear and tire geometry. Understanding the relationship between RPM, gear ratios, and tire size is the foundation of any performance tuning or fuel efficiency project.

How to Use This Tool

The Drivetrain Speed Formula

Vehicle Speed (MPH) MPH = (RPM x Tire Diameter) / (Transmission Ratio x Axle Ratio x 336)
Metric Tire Outer Diameter (inches) Diameter = ((Width_mm x Aspect_Ratio / 100) x 2 / 25.4) + Wheel_Diameter_inches

The constant 336 is derived from unit conversion: 60 minutes per hour divided by 5,280 feet per mile times 12 inches per foot times pi (approximately 3.14159). This collapses into 336.135, rounded to 336 in standard automotive practice. The formula assumes no wheel slip, no drivetrain losses, and a perfect rigid drivetrain, which is why real-world results differ.

Why Your Real Top Speed Differs from the Calculation

The simulator tells you the theoretical speed at which the engine would reach the chosen RPM in a given gear. Several real-world factors reduce the actual top speed. Aerodynamic drag grows with the square of speed, so doubling speed quadruples drag force. A typical passenger vehicle at 100 MPH is spending 50 to 70 percent of available horsepower just to push through the air. Drivetrain friction losses from the transmission, driveshaft, differential, and wheel bearings consume another 15 to 20 percent of engine power before it reaches the pavement. Finally, most modern vehicles have an electronic speed limiter in the engine control unit set between 112 and 155 MPH (180 to 250 KM/H) regardless of the mechanical capability.

Use the calculated speed as a relative comparison tool: swap between axle ratios, tire sizes, or overdrive gears and watch how the numbers change. That delta is accurate even if the absolute value is optimistic.

Frequently Asked Questions

A numerically higher axle ratio (for example, 4.10 versus 3.08) multiplies torque more aggressively, which improves low-end pull and acceleration. However, because the driveshaft rotates more times per wheel revolution, the engine must spin faster to maintain any given road speed, which lowers your theoretical top speed and hurts fuel economy at highway RPM.

Conversely, a numerically lower ratio (for example, 2.73) reduces torque multiplication, so acceleration is lazier off the line, but the engine loafs at lower RPM on the highway, increasing top speed potential and fuel efficiency. Gear ratio selection is always a trade-off between pulling power and cruising efficiency.

An overdrive gear is any transmission gear with a ratio below 1:1 (for example, 0.74:1 or 0.64:1). In a direct 1:1 gear, the driveshaft rotates once for every revolution of the transmission output. In overdrive, the output shaft actually spins faster than the input shaft, which means the wheels turn more times per engine revolution.

This lets the engine drop to lower RPM at highway speed, reducing wear, noise, and fuel consumption. Overdrive gears are standard on virtually every modern automatic and manual transmission because highway fuel economy is a critical selling point and regulatory requirement.

Your speedometer is calibrated to the original tire diameter from the factory. A taller tire has a larger circumference, so each wheel revolution covers more ground than the speedometer expects. This causes the speedometer to under-read: the car is actually moving faster than the gauge shows.

For example, upgrading from a 27-inch tire to a 30-inch tire adds about 11 percent more circumference per revolution. At a displayed 60 MPH, your true speed would be closer to 66 MPH. Running taller tires without recalibrating your speedometer also affects odometer accuracy and can alter shift points in automatic transmissions.

The standard formula is: MPH = (RPM times Tire Diameter in inches) divided by (Transmission Gear Ratio times Final Drive Axle Ratio times 336).

The constant 336 comes from unit conversion: 60 minutes per hour times 12 inches per foot divided by pi (approximately 3.14159), which converts RPM and diameter into miles per hour. For metric output, multiply MPH by 1.60934 to get KM/H.

For metric tire sizes like 275/40R20, first calculate the diameter: Diameter inches = ((Width mm times Aspect Ratio divided by 100) times 2 divided by 25.4) plus Wheel Diameter inches. For 275/40R20: ((275 x 0.40) x 2 / 25.4) + 20 = 8.66 + 20 = 28.66 inches.

The formula calculates the theoretical speed at which the engine hits the chosen RPM in a given gear, assuming no power losses. In the real world, several factors prevent reaching this figure.

Aerodynamic drag grows with the square of speed, so air resistance roughly quadruples every time speed doubles. Rolling resistance from tires and drivetrain friction also consume power continuously. Horsepower losses through the transmission, driveshaft, and differential (drivetrain losses of 15 to 20 percent are typical) reduce the power available at the wheels. Additionally, most manufacturers electronically limit top speed via the ECU for safety and tire rating compliance.

The calculated speed is best used as a relative benchmark for comparing gear and tire combinations, not as a real-world target.

This simulator calculates theoretical vehicle speeds based on mechanical drivetrain geometry only. Results do not account for aerodynamic drag, tire slip, drivetrain efficiency losses, electronic speed limiters, or any other real-world variable. This tool is for informational and planning purposes only. Always observe posted speed limits and local traffic laws.