"I'll agree, many onboard mileage numbers tend to be optimistic. On my 2015.5 duramax, it is consistently about 0.5mpg high.
so, 2-5% high. Close enough for me. On our daughters car, similar experience, somewhere around 5% high."
My understanding is that on most gas engines the computer is not able to actually measure fuel usage but they do have sensors for measuring how much air is going into the engine. It then uses a simple calculation to determine how much fuel is required to be mixed with that amount of air and considers that to be the amount of fuel used. I am wondering if the calculations are based on using pure gasoline and lose accuracy when alcohol is mixed in. You lose about 3% of the energy per gallon when burning the standard 10% alcohol mix and most reports that I seen are that the computer calculations are off about that much. Has anyone checked their computers with different amounts of alcohol in the fuel?
