Trip Display Overstates MPG

~6% variance MID vs. calculated. Believe whichever number you want. Run out of gas & you'll learn which one is accurate.

My opinion is that Toyota intentionally programmed the MID to be optimistic knowing that the vast majority of owners don't/won't keep their own records.

 

I know many people who are this way such as my parents and some of my friends. They can't be bothered to calculate their mileage. And, if their car has no trip computer, they have no idea.

In the case of my mother, her car (07 NAH) has as trip computer but she can't be bothered to learn how to use it (despite my showing her many times) so I have no idea how accurate it is. It's not that complicated: there are only two buttons and operation is just like my two previous Nissans. One button cycles thru the various displays on the trip computer and when on the MPG one (and most others), the 2nd button resets, when pressed and held.

 

My ScanGauge II average MPG is almost the same as the Prius number for each tank. They start out with different numbers, but quickly get within +-0.5 MPG over time. So I think it's accurate. Assuming the fuel injector and mileage data coming from the computer is accurate, the MPG calculation should be too. I know the speedometer reads too high, but the MPH on the SGII is accurate.

 

That is because, internally, the car knows its speed fairly accurately, and passes that accurate number along to SGII. But the speed figure displayed on the dashboard is fudged to conform to legal and industry standards.

 

My Scanguage II is about 10 MPGs low! Guess I need to reset it! In any case, only we Prii owners would sweat so much over a couple of mpg difference on a 500 mile tank!

 

Golly, at the end of the day, I don't care! It's still far better than the 18 mpg I was getting on my Venza! I'm thrilled to be getting 50+ mpg, whether its 51 or 55!

 

LOL, that's the right attitude!

But, please forgive us, there are a bunch of scientists and engineers that populate this site and we are a retentive bunch, especially when it comes to data, significant figures, units, accuracy, and precision.......and Monty Python references.....

Ni!

 

I agree that a couple of MPG invalidity is no big deal. You can enter a percentage correction factor into the Scangauge, although this is not possible with the Prius calculation. It seems to me that Toyota could solve their validity problem without encumbering liability by providing a mechanism like Scangauge II for the Prius.

 

There are many factors that cause mileage reduction in the winter/cold weather. See http://priuschat.com/forums/gen-iii...mileage-gets-worse-in-winter.html#post1411488.

 

We are no longer the nerds who say Ni.

 

I've got 7.5 years of calculated data, car PC data, and bias charted on my website - Prius Data Click the bias chart.

 

I only have 2 tanks on my 2011 Pri 11 BUT my experience is different. my 1st tank the bias was in my favor by 2 mpg. The 2nd tank was just about spot on the computer said 60.9 & calculated 60.2 but that was my fault because 1st click I then added a few mor cents in to even out my dollar amount. So, I am one of the very few who doesn't have a Prius really broken in yet with only about 1200 miles on it & the computer is not in favor of Toyota.

 

Great data!

I continue to suspect that Toyota’s MPG computation relies on calculating gasoline consumption based on fuel-injector delivery, while ignoring the delivery of gasoline vapors to the engine by fuel-tank evaporation control system. And, because the delivery by injectors is so uniquely low in the case of the Prius, the evaporation effect becomes observable.

Federal requirements (Reid Vapor Pressure) allow summer gasoline vapor pressures (Reid Vapor Pressure) as high as 9 psi, at the 100F. specified test temperature, and summer gasoline is the lower volatility blend. The Prius gasoline tank obviously does not allow itself be pressurized to this level, rather a carbon canister is used as an adsorber to hold gasoline molecules until they are later delivered to the engine when it is running. This delivery of vapors doesn’t affect engine performance since the oxygen sensors trim the fuel injector delivery to maintain the right exhaust-gas composition.

One would think that the evaporation effect could be observed in the analysis of long-term error vs. temperature, but there are other variables as well – summer gasoline contains 1-2% more energy, injector delivery varies with gasoline viscosity, gasoline blends vary regionally, etc.

 

Maybe the MPG calculation is perfectly accurate, and the odometer is wrong. :-{

 

It will be ... trust me.

You can never count your first tank because you don't know what the dealer did when he filled it. Suffice it to say that the dealer will put in the least gas possible to show a full tank.

You should fill at the same station, same pump, with the nozzle hooked on the same spring position, in low speed fill to get the most consistent fills. I've done this for 26K miles, and my calculations versus the car has been 2.2 - 2.4 MPG in favor of Toyota. I still calculate every time, but I know it's always in that realm.

I've never heard of a Prius of any Gen that was right on or in favor of the 'calculator'.

REV

 

I'm going to start keeping a running MPG average over 10 or more tanks. I'll keep a running total of the fuel consumption, and the trip odometer for total miles. Then I'll compare that to the Prius average. That's the only way to reduce variations with individual fill-ups.

 

Been there, checked it, ruled out this possibility already.

 

Right now I do use the same pump at the same gas station since I have been putting in Non Ethonal & there is only 1 station & 1 pump in our town that has that. Then again there are only like 5 stations in our 2 towns. I do go to first click & I do pump on the slow. We'll see as the Pri get's broken in.

 

I don't think it's necessarily off, at least mine isn't. Depending on how I drive the tank, I show anywhere from 44 to 52 MPG, and the amount of gasoline I add back (I always fill to the end of the filler tube) is always on the money. Heck, it's only an 11.9 gallon tank. My previous vehicle was a Hyundai Santa Fe -- that thing was awful.