a) topped off tank and only put 2-3 gallons to fill it up?
b) or you added 2-3gal at expensive gas station just to get home?
if A) than you can enter data mi/km and litter/gal and reset trip meter.
If B) write down how much gas you put, do not reset trip meter and add liters/gallons you wrote down to total. So you may end up with something like 536mi, 11.7gal entry good luck