Convert Commanded Fuel Rate (mm³) to Gallons Per Hour (GPH) & Estimated MPG
Found in logs as "Main Injection Rate" or "Commanded Fuel". Stock idle is ~8-10mm³, WOT stock ~70-90mm³.
Revolutions Per Minute of the engine.
Enter speed to calculate instantaneous MPG based on flow rate.
Fuel Consumption (GPH):–
Fuel Consumption (LPH):–
Total Injection Vol/Min:–
Instantaneous Economy:–
Understanding Duramax Fuel Rates
For Duramax owners (LB7, LLY, LBZ, LMM, LML, L5P), understanding the Calculated Fuel Rate is essential for diagnosing fuel system health and tuning performance. This metric, usually displayed on scan tools like the Tech 2, Edge Insight, or EFILive, represents the volume of fuel the ECU is commanding the injectors to fire per compression stroke.
How the Calculation Works
The Duramax is a V8, 4-stroke diesel engine. The ECU measures fuel delivery in cubic millimeters (mm³) per stroke. To convert this diagnostic number into a tangible consumption rate like Gallons Per Hour (GPH), we use the following physics:
Cylinders: 8
Combustion Events: A 4-stroke engine fires every cylinder once every 2 revolutions.
Therefore, at 2000 RPM, the injectors fire a total of 8,000 times combined per minute. If your commanded fuel rate is 60 mm³, the total volume is 480,000 mm³ per minute.
Why Convert mm³ to GPH?
1. Lift Pump Sizing: When tuning a Duramax for higher horsepower, you increase the pulse width to inject more fuel (often exceeding 100mm³). By converting this to GPH, you can determine if your stock CP3/CP4 pump or aftermarket lift pump (like AirDog or FASS) can keep up with the volume demand.
2. MPG Diagnostics: If your Calculated Fuel Rate is high (e.g., 15-20 mm³) at idle or light cruise, but your actual MPG is low, it may indicate injector balance rate issues, a boost leak, or high return rates where the ECU is compensating by commanding more fuel.
Typical Duramax Fuel Rate Values
Condition
Approximate mm³ Rate
Notes
Warm Idle
8 – 10 mm³
Depends on ECT and accessories (AC/Fan).
Highway Cruise
25 – 45 mm³
Varies by load, tires, and gearing.
WOT (Stock)
65 – 90 mm³
Varies by engine generation (LB7 vs L5P).
WOT (Tuned)
100 – 130+ mm³
Requires supporting air/fuel mods.
Note: This calculator assumes the Commanded Fuel Rate is accurate. On trucks with worn injectors (high return rates), the actual fuel leaving the tank may be higher than what is injected into the cylinder, as the CP3 has to pump more to maintain rail pressure.
function calculateDuramaxFuel() {
// Get Input Values
var mm3 = document.getElementById('mm3Input').value;
var rpm = document.getElementById('rpmInput').value;
var speed = document.getElementById('speedInput').value;
// Basic Validation
if (mm3 === "" || rpm === "" || isNaN(mm3) || isNaN(rpm)) {
alert("Please enter valid numbers for Fuel Rate (mm³) and RPM.");
return;
}
// Parse Floats
mm3 = parseFloat(mm3);
rpm = parseFloat(rpm);
speed = speed ? parseFloat(speed) : 0;
// Duramax V8 Logic (4-stroke)
// 8 cylinders. Each cylinder fires once every 2 revolutions.
// Total Injections per minute = (RPM / 2) * 8 = RPM * 4.
var totalInjectionsPerMin = rpm * 4;
var totalVolumeMM3PerMin = mm3 * totalInjectionsPerMin;
// Conversions
// 1 Gallon (US) = 3,785,411.78 mm³
// 1 Liter = 1,000,000 mm³
var gallonsPerMin = totalVolumeMM3PerMin / 3785411.78;
var gallonsPerHour = gallonsPerMin * 60;
var litersPerMin = totalVolumeMM3PerMin / 1000000;
var litersPerHour = litersPerMin * 60;
// Calculate MPG if speed is provided
var mpg = 0;
var mpgText = "Enter Speed for MPG";
if (speed > 0 && gallonsPerHour > 0) {
mpg = speed / gallonsPerHour;
mpgText = mpg.toFixed(1) + " MPG";
} else if (speed > 0 && gallonsPerHour === 0) {
mpgText = "Infinite (Coasting)";
}
// Display Results
document.getElementById('resGPH').innerText = gallonsPerHour.toFixed(2) + " gal/hr";
document.getElementById('resLPH').innerText = litersPerHour.toFixed(2) + " L/hr";
document.getElementById('resVolMin').innerText = (totalVolumeMM3PerMin / 1000).toFixed(0) + " cc/min"; // Convert mm3 to cc for readability
document.getElementById('resMPG').innerText = mpgText;
// Show container
document.getElementById('result-container').style.display = 'block';
}