ganak.appRPM Calculator
Free RPM calculator. Calculate engine RPM from speed, tire size, and gear ratio. Includes pulley RPM, SFM to RPM for machining, and torque-to-horsepower conversion. Instant results with charts.
at 60 mph
Calculation Breakdown
Detailed values from your RPM calculation
RPM at Different Speeds
Engine RPM across speeds with 3.73:1 axle ratio, 1:1 gear ratio, and 28.5" tires
Formula Reference
The equation behind this calculation mode
Formula
RPM = (Speed × Gear Ratio × Axle Ratio × 336.13) ÷ Tire Diameter
Where 336.13 = 63,360 inches/mile ÷ (π × 60 min/hr). This accounts for the conversion from linear vehicle speed to rotational engine speed through the drivetrain.
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
How RPM is Calculated
The formulas behind each calculation mode
Step 1.Engine RPM from Vehicle Speed
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 and transmission back to the crankshaft.
Step 2.Pulley Speed
A smaller driven pulley spins faster than the drive pulley. The ratio of diameters determines the speed change — a 2:1 diameter ratio means the driven pulley spins twice as fast.
Step 3.SFM to RPM (Machining)
SFM (Surface Feet per Minute) is the recommended cutting speed for a material. Converting to RPM requires the cutter or workpiece diameter so the spindle speed produces the correct surface speed at the cutting edge.
Step 4.Horsepower from Torque
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
Air compressor pulley sizing
3,500 RPM
A 1,750 RPM motor with a 6" drive pulley and 3" driven pulley:
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 for aluminum:
RPM = (600 × 12) ÷ (π × 0.5) = 4,584 RPM
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