I reset mine every tankful, and compare the results to my calculations. The computer is usually within 0.5 mpg either higher or lower than actual, and usually more accurate than that. My current mileage is up a bit incidentally since I had the piston/plug campaign performed. My current reading is 28.2 MPG on Premium, at an average 33.2 MPH, which equates to about 65-70% highway, with about 200 miles on this tankful. Never been this good with this pattern, so I'm very pleased. Hopefully, it will keep up. Also, it runs better, too.