Hey guys,when I fill up I get all diff milege from the computer,why is that.

It'* recalculating for the added fuel in the tank. The computer adjusts to that and changes based on the info the fuel sender is putting out with the added fuel.

I'm almost a rocket scientist.So I'm probably wrong, but close to being right.

why do i get diff numbers with diff grades of fuels?

Diffrent fuels have diffrent ratings as far as economy go.They may be a smidge more ethanol in one over another..Altho you should only be using 93 octane or higher is you SSEI..but fuel makers all make thir stuff a little diffrent not so sure its enough for your car to pick up on.idk i think the system in your car you are refferring to is just there to kinda give you an idea not an EXACT

along with what he said there are many other factors that go into mileage. i get the same gas from the same place, take the same way to and from work, and drive the wifes car on the weekend and my mileage still varies sometimes 2-3 mpg every tank. depends on driver, outside temp, wind, traffic, tire pressure, etc...

this might be relevant. if not just disregard this post.

https://www.gmforum.com/general-gm-chat-88/how-does-my-car-determine-how-much-fuel-ive-burned-just-curious-300725/

also one thing to keep in mind it depends on your average MPG, it does the calculations based on that