Calculate the theoretical top speed based on gearing, motor KV, and voltage.
2S = 7.4V, 3S = 11.1V, 4S = 14.8V
Check your car's manual (often 2.4 – 2.8 for off-road)
Theoretical Top Speed (MPH)0 mph
Theoretical Top Speed (KM/H)0 km/h
Final Drive Ratio (FDR)0:1
Motor RPM (Unloaded)0
Rollout (mm per revolution)0
Understanding RC Car Speed Calculation
Whether you are racing competitively or simply bashing in the backyard, knowing how to calculate the top speed of your Radio Control (RC) car allows you to optimize performance and prevent overheating. This calculator uses the fundamental physics of electric motors and gear reduction to estimate theoretical velocity.
Key Variables Explained
Motor KV: This rating represents "RPM per Volt." A 3000KV motor spins at 3,000 RPM for every 1 volt applied (unloaded). High KV is generally for speed, while low KV provides more torque.
Battery Voltage: The total electrical potential driving the motor. A standard LiPo cell is nominally 3.7V (charged to 4.2V). Common configurations are 2S (7.4V), 3S (11.1V), and 4S (14.8V).
Gearing (Pinion & Spur): The Pinion is the small gear on the motor shaft, and the Spur is the larger gear it drives. Changing these alters the gear ratio. A larger pinion increases top speed but reduces acceleration (and increases motor heat).
Internal Gear Ratio: Most RC cars (especially buggies and trucks with differentials) have an internal reduction inside the transmission case. This number is fixed and can be found in your user manual.
Tire Diameter: Larger tires cover more ground per revolution, increasing top speed but straining the drivetrain. This is often referred to as "tire ballooning" at high speeds, which can actually increase speed further than calculated.
The Math Behind the Speed
To calculate the speed manually, we follow a specific sequence of formulas:
Calculate Motor RPM:KV Rating × Voltage = Total RPM
Calculate Final Drive Ratio (FDR):(Spur Teeth ÷ Pinion Teeth) × Internal Ratio = FDR
Calculate Speed: Multiply the Wheel RPM by the Circumference to get millimeters per minute, then convert to Miles Per Hour (MPH) or Kilometers Per Hour (KM/H).
Real World vs. Theoretical Speed
Please note that this calculator provides the theoretical top speed. In the real world, several factors reduce this number, typically by 10% to 20%:
Air Resistance (Drag): As speed increases, drag increases exponentially.
Friction: Bearings, drivetrain inefficiency, and tire rolling resistance.
Voltage Sag: Under load, batteries cannot maintain their peak voltage perfectly.
Motor Efficiency: Electric motors are not 100% efficient; heat loss reduces power output.
To account for this, subtract approximately 10-15% from the result shown above for a realistic expectation on asphalt.
function calculateSpeed() {
// Get Input Values
var kv = parseFloat(document.getElementById('motorKv').value);
var volts = parseFloat(document.getElementById('batteryVoltage').value);
var pinion = parseFloat(document.getElementById('pinionGear').value);
var spur = parseFloat(document.getElementById('spurGear').value);
var internal = parseFloat(document.getElementById('internalRatio').value);
var tireMm = parseFloat(document.getElementById('tireDiameter').value);
// Validation
if (isNaN(kv) || isNaN(volts) || isNaN(pinion) || isNaN(spur) || isNaN(internal) || isNaN(tireMm)) {
alert("Please fill in all fields with valid numbers.");
return;
}
if (pinion <= 0 || spur <= 0 || internal <= 0 || tireMm <= 0) {
alert("Gear counts and dimensions must be greater than zero.");
return;
}
// 1. Calculate Motor RPM
var motorRpm = kv * volts;
// 2. Calculate Gear Ratios
// Transmission Ratio = Spur / Pinion
var transmissionRatio = spur / pinion;
// Final Drive Ratio = Transmission Ratio * Internal Ratio
var fdr = transmissionRatio * internal;
// 3. Calculate Wheel RPM
var wheelRpm = motorRpm / fdr;
// 4. Calculate Tire Circumference in mm
// C = d * pi
var circumferenceMm = tireMm * Math.PI;
// 5. Calculate Speed
// Speed in mm per minute
var speedMmPerMin = wheelRpm * circumferenceMm;
// Convert to KM/H
// (mm/min * 60 minutes) / 1,000,000 mm per km
var speedKmh = (speedMmPerMin * 60) / 1000000;
// Convert to MPH
// 1 km = 0.621371 miles
var speedMph = speedKmh * 0.621371;
// Display Results
document.getElementById('speedMph').innerHTML = speedMph.toFixed(2) + " mph";
document.getElementById('speedKmh').innerHTML = speedKmh.toFixed(2) + " km/h";
document.getElementById('fdrResult').innerHTML = fdr.toFixed(2) + ":1";
document.getElementById('motorRpmResult').innerHTML = Math.round(motorRpm).toLocaleString();
document.getElementById('rolloutResult').innerHTML = circumferenceMm.toFixed(1) + " mm";
// Show results container
document.getElementById('results').style.display = "block";
}
function resetCalculator() {
document.getElementById('motorKv').value = "";
document.getElementById('batteryVoltage').value = "";
document.getElementById('pinionGear').value = "";
document.getElementById('spurGear').value = "";
document.getElementById('internalRatio').value = "";
document.getElementById('tireDiameter').value = "";
document.getElementById('results').style.display = "none";
}