How accurate is the on-board avg mpg?

This has been discussed before. The issue is that the North American Gen2 Prii have a rubber bladder in the gas tank that makes it difficult to consistently fill the tank. You have to average several tanks to get an accurate number. That taken into account, the Gen2 tends to show about 2MPG higher on the MFD than the tank fill calculation.

 

Most people suggest removing the fuel cap first and giving it a minute or two to equalize before filling and then stop at the first click. I personally get out of the car, remove the cap, pay/select fuel grade, then fill at the lowest "automatic" speed until the first click. There's been a couple times that its shut off much earlier than it should have. In those cases I slowly fill while barely holding the trigger til it gets to about what it should be (like 5 gallons for a 50MPG 250mile tank).

 

Tank fills will quickly average out over time if you are trying to accurately hand-calculate mpg.

I don't understand the compulsion to "top off" the tank by continuing to pump gas after the first normal automatic pump shutoff. This risks overfilling the tank and damaging or inhibiting the proper functioning of the carbon filter that is used to reduce evaporative emissions into the atmosphere.

 

The bladder collapses to the amount of fuel. The point was to have no 'air' in the gas tank, thus reducing the vapors emitted by the fuel system.
It was great at that, but annoyed the owners who expected some consistency in the gas gauge.

Get gas at 3 pips if your passenger will remind you for the rest of your life that you ran out of fuel.
Get gas at 2 pips if you passenger will remind you in times of stress that you ran out of fuel.
Get gas at 1 pip if your passengers will never mention running out of gas.
If you see a blinking pip, GET GAS NOW.

 

There's a reason the Gen2's gas gauge is known as the "guess gauge". Most people go more than 100 miles before the first pip disappears.

 

mine is dead on, what the problem?

 

There's a reason the Gen2's gas gauge is known as the "guess gauge" in the North American Gen2 Prii. It is the tank bladder.

In other jurisdictions the fuel gauge works just as well as the Gen III.

It is quite normal behaviour to get 100 mi, or more, on the first 'pip'. The gauge is not linear and the 'pips' do indeed disappear faster as you clock up the distance when compared to miles travelled. From my recollection, this is the same behaviour that I have had in most cars I've driven. Maybe there is some sort of psychology to it.