I think you are confused. Trip mileage is the miles traveled per tank. So if you had a tank that was 500 miles, you would enter 500 miles. Odometer reading is your running total of your vehicle mileage, so if your vehicle mileage read 68,000 miles when you filled up, you would enter 68,000 miles. When you enter odometer readings, Fuelly is expecting the next fillup to be a number higher than the last fillup. It subtracts the two readings to get the tank mileage. That is why you were getting the errors, because when Fuelly did the math, it ended up with a negative number.
I suspected that you were entering trip miles, so I had already changed the setting for you. If that's not the case, and you're using the actual vehicle odometer reading, then it needs to be changed back.
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