Hey guys, I recently got in an argument with someone about this. I understand the math related to how the fuel economy is calculated, using MAF and fuel-flow and such, but what I want to know is, how is it calculated overall? In my 2008 Silverado 2500HD with the 6.0 Vortec, when I reset the Average Economy, Trip, Miles used, etc. I find that my average economy jumps up and down, from 22.0 MPG to 7.3 MPG. My friend tells me I'm wrong. He says the fuel economy is calculated independently each mile, regardless of what the previous mileage is. However, I think that it averages all the mileage overall, unless it is reset. This might not be very clear, but I hope this helps with an answer.