Check the mileage over more than one tank. Fill the tank. record the odometer mileage. Now for the next three weeks, record every gallon of gas added to the tank. End of week three fill the tank, add this to complete all the fuel added, also record the odometer. Now compute the miles driven in the three weeks. Divide this by the total gallons used.
I use this method on all my cars. My 2001 ST consistently gets 16 to 17. 4 X 4 with the V-6.
50 - 50 city and highway. I do not rely on the onboard computers for factual gas mileage. Trying to get an accurate number with one tank allows for error.
Hope this helps.