On my 03 with the nav screen the calculated tank average is off by about 0.2 MPG fairly consistently. I always do my own calculations by dividing miles traveled by gallons filled every time I fill up and compare it to the cars computer calculations. Surprisingly, my GS is off by less than 0.5. On my RX330 it used to be off by as much as 2 MPG.

 

That would be my observation as well. My NAV readouts for mpg are fairly conservative and my calculated mpg is usually anywhere between .2-.7 mpg better (depending on how much more highway driving I do), but that's not suprising given the ecu works on a closed loop program.