you need to do the brim test

top it up, do the mileage, then top it up again and do the mileage and work it out like that

i put up a post about a year ago when my car did some funny numbers, but using the brim method i found that the computer was telling me the truth for the first part of the journey, but when i went back the other way it was telling me LIES DAMMIT!!!!!!!!!!!!!!!

i've since been otld that some cars ave mpg is measured for the previous section of mileage so you may have reset it and it showed good mileage, but after a shite period it will show you worse for the next 20 miles or so