Interesting I fill both tanks off diesel I press the odometer button to zero travelling to and back from work I get 600kays on the big tank and 300kays on the sub tank it's always accurate , no computer stuff here and dosnt matter how i drive it and im driving from city to hyways and back everyday , I guess when later on when I'm pulling the van the kays will drop with the fuel economy for sure but I can still judge it then with that button