Real mileage?

Due to the bladder tank and the "guess gauge" on the 2nd gen, you will see variations in either direction. Sometimes, calculated mileage will be below MFD values and vice versa.

To determine how accurate is one to the other, you need to track both separately and average the values of each over many tanks.

 

The bladder flexes more when warm, so an afternoon fill up will be more gas than a morning fill up, even if you did not drive any more. I am happy I am polluting less, but I never try to out guess my gas tank, I just fill it up and call that good.

 

yes.

 

Ditto all the above. One addition: it appears that the original non-touring tires are about 2% smaller than is assumed by the odometer, so actual speed and distance traveled are about 2% less than displayed, and thus displayed MPG is about 2% larger than actual. That's the difference between 49 and 50 MPG, so in any case the difference you saw is mostly explained by variation in fillups.

 

The 'fuel usage display' is the MPG measured from the prior Reset to now so was the the Reset on the MFD fuel usage display used to clear the prior tank(s) consumption rate?

But wait I use that 'feature' to track my long term MPG with the Prius.

 

Richard, wouldn't that error affect both the displayed (MFD) mileage and the calculated mileage (dividing odometer miles by gallons added) since the odometer is used in both calculations?

 

I have to assume that you did not reset the MPG indicator at your previous fill-up since 528 miles / 41 mpg = 12.9 gallons and the tank should hold at most 11.9 gallons even if the bladder stretches out to fill the entire interior space of the tank. If you did not reset the MPG indicator, then the MPG displayed is the average MPG over multiple tanks back to the last time you reset it and not the MPG for the most recent tankful. Your most recent tank is probably closer to the 48 MPG than 41, but some previous tanks must have been a bit under 41 MPG to pull down the average.

 

Good question.... certainly, tire revolutions are the basis for both measurements, but what the odometer displays is not necessarily the same value that the MPG display uses. I'd assumed that Toyota USA reprogrammed the odometer to correct for the original tire size after the car is imported, to meet legal requirements. But then why didn't they correct the MFD and the speedo also?

 

I think if you just log the odometer readings (total miles or kilometers the car's travelled, not trip meter), and the gas amount purchased, tank after tank, you will be able to make an accurate calculation of mileage.

(You can use the trip meter to do the same thing, I actually use the trip meter to corroborate my numbers, and for a quick gauge of how far the tank is going. But I prefer to rely on the overall odometer reading for doing the calc: it's too easy to accidentaly reset the trip meter, or forget to reset it.)

Your calculation may be out for an individual tank, due to problems with uniform fill, but within 2-3 tanks the errors cancel out.

In my experience: 3rd gen in-dash mpg display (actually liters per 100 km in my case) is optimistic by 6.7% (summary of calc's since beginning of this year). I believe 2'nd gen is similar, from what I've heard and read here.