when you refill the tank do you reset BOTH trip odometers?
If not, I think the computer is taking the average from the past several thousand miles you've driven.
I reset my trip odometers first, then fill the tank. When I drive away from the station, the AVG gas mileage from the previous tankful will momentarily display then reset to zero and calculate from there. I think reseting the trip odometers lets the car know that you've filled the tank.
I always reset the trip odometers before filling the tank - it is just a habit of mine. I usually pull 18-19mpg in a combo of city/highway driving. I have an '06 RX330 with about 2,700 miles on it. I use regular 87 octane gas in it.
Mark