What does the dash read? If the dash is consistently much closer to what you calculate, then the issue is in the navi.

Do you use the same gas station/pump? Try the opposite- if you always use the same station, use different ones. If you use different ones, stick with a single one. Do anything to change or identify the source of error.

I misspoke on the last post. The dash gage uses the odometer as distance input; the navi uses GPS. Navi takes a poxition fix every XX fraction of a second and connects the dots and smooths the bumps from fix to fix. Every fix can be off by up to 10 meters- compounding in some cases, averaging out in others. Any satellite dropouts and it has to guess.