Gas Mileage?

Many folks do calculate it both ways.

Trouble is that right now with the way the bladder works it's not always a guarantee that whatever you put in is what you used. The good news is that over the long-term, that figure should be accurate, but not necessarily for a single tank.

The good news is that the computer calculates the amount of gas used - and because evaporative emissions are reduced to near zero, that should be very accurate.

 

That 32 versus 48 MPG result is kinda surprising. Because of variability of the bladder inside the tank, the amount you add after a trip is not necessarily the same as the amount used during the trip. An honest gas station pump is very accurate, but for a single trip and with no hard evidence :_> I'm slightly more inclined to believe the car's estimate based on it's own fuel-flow meter. In two out of two fill ups so far, the full-tank calculation matches my car's estimate within 0.1 MPG.

 

I also find some variation between calculated and displayed mpg, but not as much as here. I suggest that the best way to compare is to calculate mpg over several tanks, and take a weighted average of the screen displays (miles * mpg displayed / total miles over the several tanks). In my case, these numbers become similar.

 

Another question that came up in a discussion (?argument??) with my wife that might affect the mileage are the various oxygenated additives such as ?MTB and ethanol. We have 10% ethanol here as an option from one station.

 

I tend to the side that the computer will be more accurate than calculated - especially when it comes down to one tank scenarios. As I mentioned before, a quick example - it's quite likely to fill only 9 gallons in the tank, but then only use 8.5 or 8 or 10 depending on how the bladder is reacting, and how the gas gauge is too.

That being said, since I enter the info into excel - I see no harm in calculating "by hand" either - and, after 13 tanks on the spreadsheet, there's a .5mpg difference. So feel free to do both, as everyone here says - in the long run, it won't matter.

-m.

 

I got an excel program for my pda. Now it's easy to just input in the numbers just after I fill up with gas. My calculations have always been lower than the car's by 1-5 mpg.

 

If you are getting consistent calculations in regards to the computer. (i.e. always 2 mpg low), then your calculated figures are accurate, and you are getting a consistent fill in the tank.

 

The Prius has a pretty good idea of how much gas it has used and how many miles it has travelled since you last reset the consumption screen. Calculating mpgs like we've done with other cars seems to be a total waste of time with the Prius. This car does it for you.

 

We are finding an equal 2 to 2+ mpg lower actual over computer estimates based on totaled numbers over 4000 miles.

 

I agree, and like to add an interesting observation: Since I am now on finishing up my 4th tank of gas (@2337 miles 1 bar left on the tank (1.2 Gallons maybe)), I noticed that it makes quite a bit of difference at what time in the day it is that you are filling up.

The last two times befor the last feel-up, it was at around 7:00 AM and the temperature was at or below 65 F. The fourth Tank fillup was also done in the morning but it was almost 78 F. that morning.

The third tank scored 56.7 on the display, and I actually got 10.001 Gallons squized with top-off, and drove 588 miles on it.
Then to my surprise when I filled my forth Tank of gas the display showed somewhat like 55.4 MPG, but the calculated value wa 52.6.

So here is my theory: at the fourth tank the bladder was already quite a lot expanded, and allowed to fill up far more then it did the time before when it had contracted quite a bit. I will force this situation probably to night, or tomorrow evening because then the temps here in TN will be in the higher 80s so the Bladder should have had all day to expand to a near maximum.

And at that point my fuel fillup should be nearly 12 gallons, while the computor will clame only an average of 57,0 or so as oppose to the lower 52,4 or what ever it will be.

I will be willing to bet that once it is reversed that the computer will show a much worse usage, but you true calculated will be then quite a bit better, since now the bladder is quite contracted, and only lets you put less fuel in it then the tank 2-3 weeks before due to the difference in actual outside temps...

Ralph