Duramaxtuner.com :: Mileage Overview

How to figure Mileage:
Hand Calculate please: Miles traveled/Gallons of fuel used = MPG

Do not use your driver information center as a reliable mileage indicator. If the truck is not stock (tires, tune, etc.) it cannot be trusted. Even in stock form this information is unreliable, especially in ’05 and older trucks.

First thing is first, it’s important to have reliable data on distance traveled and fuel used before making any comparisons. The best way to do this is to run down the highway at 65 mph on the speedometer while verifying speed against a GPS module. If the numbers match, we have a reliable speed number. If not, a correction must be applied to the trip-odometer before figuring mileage. The odometer calculates miles based on the speedometer report. Error in the speedometer equals error in the odometer.

Example – A truck that came from the factory wearing 29.5 inch 245 series tires is now wearing 285 series 33 inch tall rubber. This translates to a 12% difference in speedometer/odometer reporting. If this truck used to get 20mpg with 245’s, it will now report 17.6mpg on the same trip using the same fuel.

Reasons for under-reporting of mileage and falsely reported poor mileage:
1. Over-sized tires that have not been corrected for in the vehicle’s on board computer
2. Lowering of differential gear ratio that has not been corrected for in the vehicle’s on board computer
3. Improper recalibration of the vehicles computer

Largest contributing factors to poor (actual) mileage:
1. Speed – Speed kills mileage. As you increase speed, the power needed to maintain drive line speed increases. The major power sucker at speed is aero drag. The amount of power needed overcome aerodynamic forces is exponential – to a power of 3, meaning that if it takes 200hp to go 100mph, it takes 800hp to go 200mph!
Another Example – if it takes 25hp to run your truck down the road at 65 mph, then running the truck at 80 mph won’t take 31hp (the avg. guess), it’ll take 47hp.

2. RPM – Turning an engine takes power, the faster you turn it the more power is wasted. This power does not go to the tires, it does not get used efficiently, do not pass go. To get the best mileage the truck should be run on the lowest side its torque band, that is – the lowest RPM where the truck can still make torque. This is where it’s most efficient, meaning it costs less fuel to make power. Most tuners widen the torque band. This allows the engine to run efficiently and make more torque at lower RPM than the factory. This improves mileage. Remember though, there’s a compromise; higher torque is harder moving parts. The tune should be tempered to balance mileage improvements with parts life.

3. Tires – More rolling mass to get moving, wider aerodynamic profile. Bad, bad.

4. Lift kits- These generally dramatically increase the size of your already large aerodynamic profile. So now it takes 30hp to run your truck at 65mph and 56hp to run the truck at 80!

The moral of the story:
The Duramax seems to like 60-70mph as a compromise between RPM and aero drag on the highway. If you run big tires, a lift kit, drive faster than this window your mileage will suffer. It’ll usually suffer more than you expect; remember the aero power of 3. Keeping your truck in a healthy state of tune, making higher than factory torque at low RPM will minimize the amount of fuel needed to make power. It will not minimize the amount of power needed to drive the truck. Mileage gains can be had with tuning, but they cannot offset lift kits, tires, and speed. No part of tuning can exponentially increase the torque output of the engine by a factor of 3. To get better mileage, focus on the aerodynamic profile of the truck.

A few words about MPG guarantees:
To offer a guarantee, a tuner must be willing to bet that the user will like the tune sufficiently not to send it back. That means if the tuner guarantees a 4mpg gain and the user gets 1.5 mpg and keeps the tune, the guarantor won even though the conditions were not met. A 1.5 mpg increase with a tune is typical; a 4mpg gain is not. If the user is looking for mileage and willing to buy a mileage guarantee, the tuner does not necessarily have to worry about meeting his claim, he only needs to worry about sending the user something that won’t be sent back. Big difference!

We at Duramaxtuner.com do not guarantee mileage gains, mostly because we cannot guarantee how you use your truck. We will guarantee your truck to run and drive better than anything else on the market.