My MPG Computer Needs a Basic Math Tutor

The car did not burn 3 gallons since the last fillup; it burned about 156/40 = 3.9 gallons. See fuzzy1's details below.

 

Do not try to figure your MPG mathemically. The gas tank has a bladder in it which sometimes prevents a complete fill up. Just trust the computer readout.

Marvinh

 

Small refills can produce erratic MPG calculations even in cars without gas tank bladders.

Different fuel pump nozzles trip at different points. Different parking pads have different slopes. As a result, the 'top of tank' varies from fill to fill, and the MPG variance can be quite large when the refill is small.

The GenII bladder makes all this much worse. If you want to keep track of mileage at the pump, average together many tanks.

 

keep in mind that the prius calculates mpg based on gasoline consumed, the EPA laboratory in Ann Arbor, MI calculates mpg based on CO emissions.

 

The Gen II Prius contains a fuel tank bladder. The bladder introduces uncertainty in the tank volume, so manual calculations are unreliable unless summed over a large number of tanks.

Tom

 

I'm extremely confident that I'm of the minority in this-and-similar issues, but I have ever-and-always used my mathematical calculations to determine Mithril's "official" MPG on a tank-by-tank basis.

Yes ... I know there's a bladder.
Yes ... I know there's a lot of room for "error" as far as filling the tank with "precisely" the same amount of gasoline as the car consumed since the last fill-up is concerned.
Yes ... I know the derived / hand-calculated info can be well above or well below what the on-board computer indicates on a tank-by-tank basis.

BUT ... my reason for doing it this way is quite simple. While there MAY be questions regarding how much gasoline the car actually consumed, there is LITTLE doubt regarding the amount of gasoline I pumped! In the long run and over a period of time involving numerous fill-ups, the "variance" greatly diminishes, and - in my particular situation ... with my particular 2008 Prius - the overall, MPG numbers - as indicated by the MFD and as compared with my hand calculations - vary by less than 2%; please trust me on this ... I've kept track of this on a rather involved spreadsheet since Day One!

LASTLY ... as I've said before on several threads at this site: when the MFD starts paying for the gasoline I pump, I'll use its figures when calculating Mithril's "official" MPG rating ...

 

I bet you're still relying on the car telling you how far you went. .... and the gas station telling you how much gas it metered out (volume corrected for temperature, of course). Face it, all you really know is how much money you're charged.

BTW, I laso have spreadsheet with all purchases recorded.

 

That's EXACTLY my point ... I know how much money I've spent!

And - with a snicker and a pashaw - if you think I'm gonna trace my routes with a ruler to "confirm" distance driven ... well ... you're wrong ...

Certain things rather have to be "regarded" as being be accurate: miles driven according to the ODO is one of them ... but the MFD, in my opinion, is not.

 

I am the same.

The actual gallons pumped divided into miles on the odometer produce the only real number.....even when they produce 70.9 for you last week (as they did for me), I know I will get something like 55 MPG measured next tank (my first pip fell at 56 mi - pretty clear that was a bladder reading!)

The MFD has a bias of anywhere from 2-4 MPG high - while the measured tanks vary wildly - but are the Mills of God grinding - they eventually yield the only right answer..........

 

There are ways to check this. Without GPS (I don't trust my GPS for this because it is to susceptible to jumping around, adding extra distance to the trip reading.)

Does your state have some Speedometer / Odometer check stations along the highways -- 5 miles of prominently displayed & accurate mileposts?

I prefer the standard highway mileposts over longer distances, though these require vigilance and care. While most of these are set by the original highway surveys, there can be a number of unmarked 'busts' where realignments have changed the original road distance, some signs are placed out of position for visibility or obstructions, and divided highways with large separations can have different lengths in opposite directions. So this method requires many observations along a route, watching for patterns and discarding inconsistent sections.

My Prius odometer reads slightly under, by a few tenths of a percent.

 

Sigh . . . I'm just gonna go for a nice drive.

 

As indicated above ... thanks to fuzzy1 for the input / advice.

It's been a while since I've visited Prius-Chat, and even longer since I last made a lengthy road trip - we don't drive much ... Mithril is 21 months old and JUST crossed the 16k threshold - that involved stretches of interstate with mile markers ... but will be doing so later on this month. I'll try my best to do a "check" of the odo vs. mile markers over a ten to twenty mile stretch and see how accurate it seems to be. I'll try to remember to come back to this thread and post the results of the "check" once I've done it.

 

Is there a reason for the fuel bladder? Kinda makes me wonder if Toyota put it there just to thwart us from computing "actual" gas mileage so we have to trust the MFD

At any rate, I'm looking forward to comparing my "at-the-pump" mileage to the MFD mileage over the lifetime of my Prius. I've been doing the "at-the-pump" calculation since I started driving almost 14 years ago, and have no intention of stopping. No spreadsheets though -- now I use an app on my phone that tracks "at-the-pump" mileage for both of my vehicles per tank, and over time, and also tracks a per-tank and lifetime average of cost per mile as it fluctuates with fuel prices (another number I'm looking forward to tracking on my Prius!).

 

It minimizes air contact with the fuel, reducing evaporative losses. The new 2010 Prius eliminated the fuel bladder.

Tom