But very rarely! The Prius has been both very close, and way off. The last 2 tanks were just under 70 mpg, it guessed just over 70 mpg. However, it has also guessed 70 when 62 was the real figure. I must have had 50 fill ups in the i20, only a couple of them I remember stating a lower guess on the computer. I have to say, when it comes to L/100KM though it may as well be pints per furlong - my head only computes in miles per (imp.) gallon...
Generation II (2004-2009) Prius owners have another issue that really affects onboard computer versus "real life" calculations. Our cars are fitted with flexible fuel bladders inside the fuel tanks. Depending on the flexibility of the bladder at any given fill the tank capacity may vary by as much as 3 gallons, even when filling at the same station, same pump and same rate of fill. My onboard mileage will usually be off +/- 2 or 3 mpg but it has been off by as much as 6 mpg.
I glance occasionally at the onboard readings to get a sense of how I'm driving at the time but I don't expect specific accuracy. The same with actual calculations at the pump. I depend on averages over multiple tanks to check the health of the car. I don't get excited about any individual fill.
I just take the fuel fill receipt and record the miles traveled. You can travel miles without consuming fuel if you EOC. You can consume fuel without travelling any distance if you allow your engine to idle. I love EOC and hate idling. Fuelly just allows me to publicly display my mileage and keep track of that mileage over time, while confirming my math is correct, a skill I acquired when calculators weighed 20 pounds and weren't allowed in school.
To check your difference between two different tires if you replace them, or even if you just want to know.
Mark a point on the tire and the pavement, roll the car until the mark covers one revolution precisely, then mark the pavement again. Measure the distance between the two marks with your preferred measurement type.
A 25 inch diameter tire will roll right at 3.1416X25 inches which is 78.54 inches (per revolution). A miles is 63,360 inches. 63360/78.54=715.6 revolutions per mile.
Lets say you put slightly larger diameter tires on your car. They roll 705 revolutions per mile. Since they roll fewer revolutions per mile your speedo (and odometer) will register lower at identical speeds. To calculate the difference divide 715.6/705=1.015. Multiple the distance recorded on your odometer by 1.015 and you get the true distance.
Works fine or everything but ice, metric, US, Imperial, just require adjustments.