The trip computer is good to within 3 or 4 mpg, judging on tank-to-tank fills (even that's a bit variable, a slow fill or the car not level makes several litres difference to the max fill).
My usual commute is 15miles each way on country roads 40mph, which gives a trip computer reading about 50mpg, which tallies with tank-to-tank fill-ups.
I swapped cars with my wife last Nov/Dec for 8 weeks or so - and I was surprised that the consumption dropped to about 35mpg, on her 3-4mile urban trips. She is a gentle driver though tends to stick in the low gears longer than she needs.