When there is no excess friction from the brakes, I find the gom is pretty accurate. As I have said in this post, most of my driving is on country roads and I consistently drive at around 40mph and rarely more than 50mph. I have eco turned on all the time and I don't thrash the car. Yesterday I did 4 trips totalling 55 miles and the gom dropped 54 miles. This is what I experienced ever since having the car (apart from when I had the problem with something binding). Where I guess it wouldn't be accurate is if you did a 50 mile journey one day at around 40mph and then expected the gom to be accurate the next day when you drove to 80mph with eco off.