How do you calculate RPM from vehicle speed?

RPM = (Speed in mph × Transmission Gear Ratio × Axle Ratio × 336.13) ÷ Tire Diameter in inches. The constant 336.13 converts mph to inches per minute divided by π. For example, at 60 mph with a 3.73 axle ratio, 1:1 (4th) gear, and 28.5" tires: RPM = (60 × 1.0 × 3.73 × 336.13) ÷ 28.5 ≈ 2,640.

How do I calculate speed from RPM?

Vehicle Speed (mph) = (Engine RPM × Tire Diameter in inches) ÷ (Transmission Gear Ratio × Axle Ratio × Transfer Case Ratio × 336.13). This is the inverse of the RPM formula. For instance, 2,500 RPM with 28.5" tires, 3.73 axle, and 1:1 transmission gives approximately 57 mph. Switch to the RPM to Speed mode in our RPM to Speed Calculator to compute this instantly.

How do I calculate gear ratio with RPM?

Required Gear Ratio = Input RPM ÷ Desired Output RPM. For example, if a motor spins at 1,750 RPM and you need 500 RPM at the output shaft, the required reduction ratio is 1,750 ÷ 500 = 3.5:1. You can also use tooth counts: Output RPM = Input RPM × Driving Teeth ÷ Driven Teeth. Our Gear Ratio Calculator handles these calculations.

How do I calculate pulley RPM from pulley sizes?

Driven Pulley RPM = (Drive Pulley Diameter × Drive Pulley RPM) ÷ Driven Pulley Diameter. If the driven pulley is smaller than the drive pulley, it spins faster (speed increase). If the driven pulley is larger, it spins slower (torque increase). Belt speed in FPM = (π × Pulley Diameter × RPM) ÷ 12. Our Pulley RPM Calculator computes driven RPM, required pulley diameter, and belt speed.

How do I convert RPM to feet per minute or SFM?

SFM (Surface Feet per Minute) = (π × Diameter in inches × RPM) ÷ 12. For machining, RPM = (SFM × 12) ÷ (π × Diameter in inches). To convert SFM to m/min, multiply by 0.3048 (1 SFM = 0.3048 m/min). For pulley belts, belt speed in FPM uses the same formula with the pulley diameter. Use our SFM to RPM Calculator for quick conversions.

How do I calculate RPM from gear ratio?

Engine RPM = Driveshaft RPM × Transmission Gear Ratio. The driveshaft RPM itself depends on vehicle speed, tire diameter, and axle ratio. To get the full picture, you need all three values: transmission ratio, axle ratio, and tire size. Our calculator handles all of these together.

What is the formula for pulley RPM?

Driven Pulley RPM = (Drive Pulley Diameter × Drive Pulley RPM) ÷ Driven Pulley Diameter. If the driven pulley is smaller than the drive pulley, it spins faster (speed increase). If the driven pulley is larger, it spins slower (torque increase). The ratio of diameters equals the inverse of the speed ratio.

How do you convert SFM to RPM?

RPM = (SFM × 12) ÷ (π × Diameter in inches). SFM (Surface Feet per Minute) is the recommended cutting speed for a material. For example, aluminum typically requires 600–1000 SFM. A 1-inch end mill at 600 SFM: RPM = (600 × 12) ÷ (3.14159 × 1) = 2,292 RPM.

How do you calculate horsepower from torque and RPM?

Horsepower = (Torque in lb-ft × RPM) ÷ 5,252. The constant 5,252 comes from 33,000 ft·lbf/min (the definition of 1 HP) divided by 2π. This means that torque and horsepower are always equal at exactly 5,252 RPM.

What RPM should a car engine run at highway speed?

Most modern cars cruise at 1,500–2,500 RPM at highway speed (60–70 mph) in their highest gear. Vehicles with overdrive transmissions (0.6–0.7 ratio) and moderate axle ratios (3.0–3.5) tend to run at lower RPM for better fuel economy. Trucks and performance cars with higher axle ratios (3.73–4.10) run at higher RPM.

How does tire size affect engine RPM?

Larger tires reduce engine RPM at any given speed because they cover more ground per revolution. A tire diameter increase from 28" to 31" (about 10.7%) reduces RPM by approximately 9.7% at the same speed. This is why lifting a truck with larger tires often requires re-gearing the axle ratio to maintain drivability.

What is the difference between engine RPM and driveshaft RPM?

Engine RPM = Driveshaft RPM × Transmission Gear Ratio. In direct drive (1:1 gear, typically 4th), engine and driveshaft RPM are identical. In 1st gear (ratio ~3.5:1), the engine spins 3.5 times for every driveshaft rotation. In overdrive (~0.7:1), the driveshaft actually spins faster than the engine.

How do I calculate RPM for a lathe or CNC machine?

For turning operations, RPM = (SFM × 12) ÷ (π × Workpiece Diameter in inches). Look up the recommended SFM for your material (e.g., mild steel = 80–100 SFM with HSS tooling, 400–600 SFM with carbide). As the workpiece diameter decreases during facing, RPM should increase to maintain constant SFM.

What does the 336.13 constant mean in the RPM formula?

336.13 = 63,360 inches per mile ÷ (π × 60 minutes per hour). It converts linear speed in miles per hour to the number of tire revolutions per minute, accounting for the tire circumference (π × diameter). This single constant replaces three separate unit conversions in the RPM formula.

RPM Calculator

Speed Unit

Diameter Unit

Vehicle Speed (mph)

Tire Diameter (inches)

Axle (Rear End) Ratio

Transmission Gear Ratio

Transfer Case Ratio

Tire Size Code (optional)

Engine RPM

2,640RPM

at 60 mph

Calculation Breakdown

Detailed values from your RPM calculation

Driveshaft RPM

2,640

Tire Circumference

89.54"

Revolutions / Mile

708

Engine RPM

2,640

RPM at Different Speeds

Engine RPM across speeds with 3.73:1 axle, 1:1 trans, and 28.5" tires

Engine RPM

Driveshaft RPM

Formula Reference

The equation behind this calculation mode

Formula

RPM = (Speed × Trans Ratio × Axle Ratio × T-Case × 336.13) ÷ Tire Diameter

Where 336.13 = 63,360 inches/mile ÷ (π × 60 min/hr). The transfer case ratio (default 1.0) accounts for 4Lo gearing in 4WD vehicles. This traces the path from the tire contact patch through the differential, transfer case, and transmission back to the crankshaft.

What is RPM?

Understanding revolutions per minute

RPM (Revolutions Per Minute) measures how many complete rotations a shaft, wheel, or other rotating object makes in one minute. It is the standard unit for rotational speed in automotive, industrial, and machining applications. Whether you are tuning an engine, sizing pulleys, or setting spindle speeds, RPM is the key measurement that connects linear motion to rotational motion.

Automotive

Engine and driveshaft RPM from vehicle speed, tire size, and gear ratios

Pulleys & Belts

Calculate driven pulley speed from drive pulley diameter and RPM

Machining

Convert SFM to spindle RPM for lathes, mills, and CNC machines

Gear Ratio RPM Calculations

Find output speed, required ratios, or RPM from tooth counts

Gearing lets you trade speed for torque — or vice versa. Use the Gear RPM mode to solve three common problems without manually rearranging formulas. For compound gear trains, multi-stage reductions, bicycle gearing, and vehicle speed from RPM, try the full Gear Ratio Calculator.

Output RPM from Ratio

Output = Input ÷ Ratio

Enter input RPM and gear ratio to get the output shaft speed.

Example: 1,750 ÷ 3.5 = 500 RPM

Required Ratio from RPM

Ratio = Input ÷ Output

Know your input and target output? Solve for the gear ratio you need.

Example: 1,750 ÷ 500 = 3.5:1

RPM from Gear Teeth

Output = Input × Drive ÷ Driven

Working with physical gears? Enter tooth counts directly.

Example: 1,800 × 20 ÷ 60 = 600 RPM

RPM to Speed and MPH

Convert engine RPM to vehicle speed for any drivetrain setup

Core Formula

Speed (mph) = (RPM × Tire Dia.) ÷ (Trans Ratio × Axle Ratio × T‑Case × 336.13)

The constant 336.13 = 63,360 inches/mile ÷ (π × 60). For km/h, multiply the mph result by 1.609.

Gear & Tire Planning

See how axle ratio swaps (3.08 → 4.10) or larger tires change your highway RPM. Pair with the Tire Size Calculator to compare fitments side‑by‑side.

Speed at RPM Chart

The bar chart shows engine and driveshaft RPM from 10–80 mph (20–160 km/h) for your exact gearing — spot your cruising sweet spot instantly.

4WD / 4Lo Crawl Speeds

Enter a transfer case ratio (commonly 2.72:1) to see true engine RPM when crawling in low range. Type a tire code like 265/70R17 to auto‑fill diameter (~31.6″).

Quick Inverse Checks

Found your RPM? Switch to RPM → Speed mode to verify — the same tire, axle, and gear inputs should return your original speed within rounding tolerance.

How RPM is Calculated

The formulas behind each calculation mode

Step 1.Engine RPM from Vehicle Speed

RPM = (Speed × Trans Ratio × Axle Ratio × T-Case × 336.13) ÷ Tire Diameter

The constant 336.13 converts from miles per hour to inches per minute, divided by π. This formula traces the path from the tire contact patch through the differential, transfer case, and transmission back to the crankshaft. For a deeper dive into gearing, see our Gear Ratio Calculator.

Step 2.Gear Ratio RPM

Output RPM = Input RPM ÷ Gear Ratio (or × Driving Teeth ÷ Driven Teeth)

Gear trains multiply or divide rotational speed in proportion to tooth counts. The output torque is inversely proportional to the speed change. A 4:1 reduction quadruples torque while cutting speed to 25%.

Step 3.Pulley Speed & Belt FPM

Driven RPM = (Drive Diameter × Drive RPM) ÷ Driven Diameter

A smaller driven pulley spins faster than the drive pulley. The ratio of diameters determines the speed change. Belt surface speed (FPM) = (π × Pulley Dia × RPM) ÷ 12 — the same on both pulleys.

Step 4.SFM to RPM (Machining)

RPM = (SFM × 12) ÷ (π × Diameter in inches)

SFM (Surface Feet per Minute) is the recommended cutting speed for a material. 1 SFM = 0.3048 m/min. Converting to RPM requires the cutter or workpiece diameter for proper spindle speed at the cutting edge.

Step 5.Horsepower from Torque

HP = (Torque in lb-ft × RPM) ÷ 5,252

The constant 5,252 = 33,000 ÷ (2π). Torque and horsepower always cross at 5,252 RPM on a dyno chart — below that RPM, torque is numerically higher; above it, HP is higher.

Common Mistakes to Avoid

Errors people make when calculating RPM

Confusing tire diameter with rim size

A 17" rim does not mean a 17" tire diameter. The overall diameter includes the sidewall height on both sides.

Forgetting the transmission ratio

Using only the axle ratio gives you driveshaft RPM, not engine RPM. Multiply by the current gear ratio for engine RPM.

Using radius instead of diameter

The SFM formula and pulley formula both use diameter, not radius. Using radius will give you exactly half the correct answer.

Mixing metric and imperial units

Tire diameter in mm and speed in mph will produce wrong results. Convert to consistent units before calculating.

Real-World Examples

Practical applications of RPM calculations

Highway cruising RPM

1,784 RPM

A truck with 3.73 axle ratio, 0.63 overdrive (6th gear), and 31" tires at 70 mph:

RPM = (70 × 0.63 × 3.73 × 336.13) ÷ 31 = 1,784 RPM

Plan axle & gear setups →

Gearbox output RPM from ratio

500 RPM

A 1,750 RPM electric motor driving through a 3.5:1 reduction gearbox:

Output RPM = 1,750 ÷ 3.5 = 500 RPM

Open Gear Ratio Calculator →

Air compressor pulley sizing

3,500 RPM

A 1,750 RPM motor with a 6" drive pulley and 3" driven pulley, belt speed ~2,749 FPM:

Driven RPM = (6 × 1,750) ÷ 3 = 3,500 RPM

Milling aluminum at 600 SFM

4,584 RPM

Using a 0.5" end mill at the recommended 600 SFM (183 m/min) for aluminum:

RPM = (600 × 12) ÷ (π × 0.5) = 4,584 RPM

Try Feeds and Speeds Calculator →

Dyno power from torque reading

300 HP

An engine making 350 lb-ft of torque at 4,500 RPM:

HP = (350 × 4,500) ÷ 5,252 = 300 HP

Frequently Asked Questions

Common questions about RPM calculations for automotive, pulley, machining, and power applications

Engine RPM = (Vehicle Speed in mph × Transmission Gear Ratio × Axle Ratio × 336.13) ÷ Tire Diameter in inches. If you have a transfer case (4WD), multiply by the transfer case ratio as well. For example, at 60 mph with a 3.73 axle, 0.70 overdrive transmission, and 31" tires: RPM = (60 × 0.70 × 3.73 × 336.13) ÷ 31 ≈ 1,698. Use the Speed to RPM mode on our RPM Calculator for automatic calculation.

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Last updated Jun 14, 2026

RPM Calculator

Calculation Breakdown

RPM at Different Speeds

Formula Reference

What is RPM?

Gear Ratio RPM Calculations

RPM to Speed and MPH

How RPM is Calculated

Common Mistakes to Avoid

Real-World Examples

Frequently Asked Questions

Embed RPM Calculator

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