how to fine-tune mpg display

[kaspr]

I have an '03. I understand that these vans use fuel injector pulses to calculate fuel usage for the gas mileage readouts. My van overstates the fuel used by 11% (I've heard that most of these vans understate the fuel usage, but mine overstates). The result is that if my display says 19 mpg, I'm actually getting 21 mpg. Is there any way to calibrate this so it's accurate?

Thanks.