Maybe some of you Prius veterans can explain this for me. I've always assumed that my fuel and mileage reports on myelectronic display were correct, but I'm having second thoughts. Yesterday I filled up after a 500+ mile tank. (Yes, I was down to one pip, beeping and flashing, so I was a little nervous.) The fuel usage display read 41 mpg. The miles were 528. The amount of gas I put in the tank was 10.something (let's call it 11 just to be conservative). When I divide 28 by the 11 gallons I put in the tank I come up with 48 mpg, which is way better than what my display said. Is this common? Or, to put it another way, should I be gloating way more than I currently am? Thanks.
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.
There are many threads discussing this in great detail. I'll try to give a generic overview. Like a balloon, the bladder holds more when it's warm because it can stretch more. Just like the guys said. So this means there are capacity variations between fill-ups. This means you can't gauge your mileage based on the refill. Rather, you have to gauge it on the actual number of miles driven on the actual amount of gas in the tank from the previous fill-up. Unfortunately, the only way to be 100% certain is to use all the gas in the tank every time (please don't run out gas in yor Prius). Since there is always a varying amount of gas left in the tank, you don't know for certain exactly how much Gas you actually used. So, in summary, you the driver never really know exactly how much gas you used to travel a certain number of miles. This makes hand math very difficult. However, the car knows how much fuel is being used at all times with relatively good precision. So, within a pretty tight tolerance level, your best bet is to use the car's reported mileage.
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.
