I track every tank and record the car display, miles, and gallons input. I've tracked and charted the difference across 7.5 years and two Prius vehicles. Web site Large pdf chart The data shows the display indicates 1.4mpg bias better than the actual mileage. That's about a 2.6% error. The display is consistant, but has a bias. The miles/gallons calc will vary tank to tank due to the bladder.