Economy Tuning

Here in the wonderful state of WA diesel just hit $6 per gallon. I have been wanting to work on a fuel economy tune for a while to really maximize fuel energy. I guess now is good time lol. I have been considering what elements would have the greatest effect on economy, this being first off timing to maximize heat energy, and transmission shift and lock up strategy, to keep the engine in its most energy dense RPM. I have played with timing calculators, and im wondering if I use a "stock" file and effectively set the cruise area of the table at 50% ish SOI to really maximize the cruise CFR? Im sure there are other helpful factors, and I may be way off but am looking for advise on what would be the correct direction to achieve best results. This would be a unladen scenario.

Jake

Here's a post I wrote years back on mileage. It's probably not exactly what you're looking for - but it's worth reading.

DURAMAXTUNER.COM :: MILEAGE OVERVIEW

How to Calculate Fuel Mileage

Always calculate mileage manually:

MPG = Miles Traveled ÷ Gallons of Fuel Used

Do not rely on the driver information center (DIC). If the truck is modified (tires, gearing, tuning, etc.), the displayed mileage is unreliable. Even stock trucks—especially 2005 and older—often report inaccurate data.

Start with Accurate Data

Before comparing mileage, ensure your distance and speed are correct:

Drive at a steady speed (e.g., 65 mph indicated)Verify actual speed using GPSIf the readings don’t match, apply a correction factorThe odometer is based on the speedometer.If speed is wrong, distance—and therefore MPG—is wrong.

Example

A truck originally equipped with ~29.5” tires (245 series) is upgraded to ~33” tires (285 series):

~12% increase in tire diameterSpeedometer and odometer under-report by ~12%If the truck previously achieved 20 MPG, it will now appear to get:

20 MPG × 0.88 = 17.6 MPG (reported)

No actual loss in efficiency—just bad data.

Common Causes of Incorrect (Under-Reported) Mileage

Oversized tires without recalibrationGear ratio changes without recalibrationImproper ECU/TCM calibration

Primary Drivers of Poor Actual Mileage

1. Speed (Biggest Factor)

Aerodynamic drag increases exponentially with speed (roughly proportional to velocity³).

If it takes 200 HP to go 100 mphIt takes ~800 HP to go 200 mphReal-world example:

65 mph → ~25 HP required80 mph → ~47 HP required (not ~31 HP)👉 Small increases in speed = large increases in fuel consumption

2. RPM

Higher RPM = more internal friction and parasitic loss.

For best efficiency:

Operate at the lowest RPM within the usable torque bandThis is where the engine produces power most efficientlyWell-designed tuning can:

Broaden the torque curveImprove low-RPM efficiencyIncrease usable torque⚠️ Trade-off:Higher torque increases stress on components. Balance performance with durability.

3. Tires

Increased rotational massLarger contact patchGreater aerodynamic dragAll reduce efficiency.

4. Lift Kits

Lifted trucks increase frontal area and drag significantly.

Example:

65 mph → ~30 HP required80 mph → ~56 HP required

Key Takeaways

The Duramax platform is most efficient around 60–70 mphFuel economy suffers quickly when you:

Increase speedAdd large tiresInstall lift kitsTuning can improve efficiency, especially at lower RPM, but:

Tuning cannot overcome aerodynamic drag, gearing, or physics.

On MPG Guarantees

Mileage guarantees are often misunderstood.

A tuner offering a “4 MPG gain” is effectively betting:

The customer will like the tune enough not to return itIf the user gains 1.5 MPG and keeps the tune:

The guarantee is technically “successful”Even though the claim wasn’t met👉 A typical, realistic gain from tuning alone is ~1–2 MPG👉 4 MPG gains are uncommon without other changes

Our Position

At DuramaxTuner.com:

We do not guarantee mileage gains, because we cannot control how the truck is usedWe do guarantee improved drivability, performance, and overall operation

Excellent writeup, the gist I got is drag and driving style contribute to greater gains than can be had from tuning? Makes sense. And to be honest, we are talking about a 8600 lbs 1 ton that is built to tow, economy is not its primary function.