WHAT how do i know my MPG

well its not that hard, it might take some extra money, but it will not be wasted.
FIRST:Fill up your tank ALL THE WAY. let it fill up until the pump shuts off itself, wright down how much gas it took, not the dolor amount(unless you want $ per mile) but the number of galons it took
SECOND:push the trip counter button
THIRD:drive
LAST:when you are about to run out of gas(the light comes on when you have about 20 miles left) wright the miles you have traveled, and devide your gas bought in step 1 by total miles, that is your MPG