Any way to adjust MPG calculation/display?

[WS6FirebirdTA00]

I tried to search without a lot of luck, maybe because there is nothing you can do.

 

My MPG on the dash has been consistently off 1.8-2.2 MPG vs actual calculated numbers. I filled up today and it showed 19.9 MPG on the display (reset every fill up) and calculated out to 17.7 MPG using the same pump and same fill process.

 

In my old BMW I could go in the computer setting through the dash to calibrate. Wondering if I or the dealer may have access to do the same. Being off that much makes it pretty much useless.

 

I am not sure if the range keeps some fuel in reserve for the calculation, but that seems to be 100% right adding that to my current mileage and dividing out.