total travel summary when you power off the car

It is an estimate at best.
In order to get that number, you must have done a majority of your miles in EV mode.

 

Yeah, it's accurate... in a sense. It calculated total gasoline used over the distance traveled, so there is no accounting for EV vs HV miles, just like the trip odometer. I find this really annoying. If I drive a few miles and use EV the entire time, my gas mileage is displayed at 999mpg. This calculation method is not helpful for someone like me, who is more interested in the mileage achieved when I'm actually using gas.

Your best bed for an accurate measurement is to reset the trip odometer when you go to HV mode and check your fuel mileage that way.

 

They're already giving you that information by giving you MPG. If you traveled 40 miles and the display shows 93 mpg, that means 0.43 gallons of gas was used.

 

It's not accurate because the speedometer is not accurate. Due to the inaccuracy of the speedometer being off by +-3 mph, that translates to inaccurate total miles & mpg on the power off display along with both trip A and trip B. That gives the driver a false sense of accomplishment when it shows you did 92mpg when you actually did around 85mpg. That's why my hand calculated mpg is always lower than what is displayed on the trip monitors. Toyota went out of their way to program the inaccurate speedometer but made sure the odometer was correct otherwise they would be hit with lawsuits.

 

The speedometer is calibrated to read a little bit "fast", so people can't use "the speedometer said I was under the speed limit" defense. People with OBD-II tools can see that the speed information on the data bus is accurate, but the speedometer reads fast. The odometer is legally required to be as accurate as possible. It isn't just an issue of lawsuits; the government would be on their case otherwise.

 

The other interesting side effect that I've noticed with the '14 PiP (and not in my previous '04 and '11 Prii) is that setting the cruise control to 70mph results in the speedometer showing 71mph. Setting for 75mph results in the speedo showing 76 or 77mph.

I'm almost thinking of asking the dealer to look at that, as it's an inconsistency (displayed speed > setpoint on cruise) I've never seen in any Toyota (including the aforementioned Prii and multiple Camrys and Avalons).

Can anyone else check to see if this is common or if mine has a unique issue?

 

Why would Toyota use the speedometer to calculate miles driven when there's already an odometer reading available for that purpose?

 

One week I drive my Pip then next I take my wife's Acura TSX same exact trip, the trip mileage on the PIP is always 3 miles more than the 90 miles on the TSX.

 

Trip mileage as displayed at the end of the ride, or trip mileage as registered on the odometer?

 

With the PiP's stock tire, the difference between a worn tire and a new one is close to 3%*


*Stock tire 185/70/15, rolling circumference of 79.12in compared to a rolling circumference of 76.93 based on the loss of about 0.34in/8.85mm of tread depth gives 2.8% difference.. but this also ignores that not all tires are the same rolling circumference even if the tire size is the same

 

I notice the speedometer on my 2014 PIP is a little off. It's showing 2,3 MPH faster than what it really is. I ride motorcycles and it's pretty common for Japanese bike to do this also.

But odometer shouldn't be off. I have no way to measure that... I remember my 2011 corolla show 17-18 miles from home to work and my Prius is showing the same thing I think, I will check that again next time.

 

The accuracy of the mpg has nothing to do with the speedometer accuracy, and everything to do with Toyota wanting to fudge the numbers. If they made it accurate it'd still be good, so...

 

Don't know about US and Canada, but in EU there is an UN regulation UN ECE #39, §5.3. in effect. This regulation defines that every speedometer must display value greater than exact, but not by more than 10% + 4km/h. Violating this regulation would ban the vehicle sale. So Toyota choose to add a great deal of offset, to stay well in the safe spot.

I hope it affects speedo only and leaves odo and averages unfudged.

 

The MPG unit of measure is well suited for gas guzzlers, not such a fuel efficient vehicle. 999MPH represents close to "infinite".
An oz/100mi would fit PiP better!

In Japan and (most of the) EU the fuel consumption is measured instead of mileage. These are reciprocal units of measure. This explains why many people here on priuschat.com are amazed how "small" their milage is with their new PiP. What they actually mean is how small their fuel consumption is.
The unit is litres/100km and PiP displays it really nice, using one decimal number:

 

Has anybody questioned how accurate the fuel flow sensor is? The amount of fuel consumed is used as a denominator in MPG calculation. Thus the inacuracy of this value impacts the inacuracy of MPG most, when this value is small.

 

It has everything do with with the speedometer when the mpg relies on the speedometer to calculate the distance traveled which then calculates the mpg. I verified this with Torque and other vehicles. And yes, maybe Toyota wanted to fudge the numbers but I think they got lazy since it is a requirement to have the speedometer read higher in other parts of the world so then they just left it for everyone else.

 

The speedometer display is not driving the mpg calculation, it's just end-of-the-chain display. And as both you and Rozi have said, it reads high, on purpose, with the aim to get people to go a little slower.

The mpg displayed by the car would be calculated by the odometer, which must be reasonably accurate (or they'd risk a lawsuit) and fuel flow.

Anytime a result is consistently skewed to one side, it's intentional.

 

Between the computer and what I see at the pump I see a 3-4% difference. This is fairly standard among all Prii.

So in other words it is very precise but not fully accurate.