duramax Diesel question

The DIC calculates mpg based on reported fuel injection * rpm. Almost all tuners increase the pulsewidth (the amount of time the injector is open / injecting fuel) to add more power. So what happens is the reported injection and actual fuel injected are not even close, causing the DIC to report better mpg's. You have to "hand calculate" your mpg's. Fill up and reset the trip odometer and fuel consumed. drive until fuel is needed. fill up and get a recept, on the back write the miles from trip odometer, and gallons burned from DIC. When you get board go through recepts and calculate mpg's, and compare pump gallons to DIC gallons. After a few tanks you should be able to estimate your DIC is off by X% and you can correct on the fly.