I'm not sure why this is but when I calculate my mpg via "miles traveled on a tank divided by gallons replaced," I come up with consistently lower mpg than what is displayed by the car itself. I do reset this display each time I fill the tank, tracking my mpg per tank instead of lifetime.

I hope what I'm trying to say makes sense.

--jay

 

That is typical. Most people who track this report that the displayed mileage is between 1 and 2 MPG higher than actual.

 

I have always reset the odo trip meter at every fill up other wise it goes to 1000 and starts over. but i calculate from ooo to the end mileage and go from there. (Reset Reset)..

 

There are folks who absolutely believe the display is accurate and the pump/odometer readings are not to be trusted. They claim the bladder in the tank results in 'variable tank size' due to various speculations. Since I 'pay the pump', those are the numbers I use.

Bob Wilson

 

This issue comes up from time to time. My current lifetime average caculated at the pump is 61.2mpg. The car's display shows 63.5 lifetime so it is clearly a hair optimistic.

Both methods have anomalies. The display is very useful for comparing one tank to another and is the most consistant overall. The display is consistant between tanks but it is also consistantly too high.

The caculated method is accurate over time but is useless for comparing one tank to another due to fill issues. The fill issues average out over time so in the long run the caculated method seem like the way to go.

I wish the database kept track of mileage 3 ways. We have already talked about 2 ways and I wish both the displayed and calculated mileage could be entered. The third way would be a "hybrid" method that used the best of both of the other two method combined into a corrected figure. I believe that the actual best method would be to use the displayed figure (which is a good way to compare between tanks) adjusted down to match the calculated at the pump figure.

 

I've tracked mileage by miles / gallons at the pump and by the computer calculation on both my Prii - 2003 and 2005 - from the start. First off, while for a single tankful the bladder expansion might cause the calculated mileage to be off, it should be obvious that over a couple of years, dividing the miles driven by the gas you actually put in will give you the correct mileage, while the computer, which is calculating on the fly, may or may not be correct.

For my 2003 Prius, after 53 tankfuls, true mpg is 42.4, while the computer reports 46.0, or 9% high. For my 2005 Prius, after 23 tankfuls, true mpg is 47.2, while the computer reports 49.0, or 4% high.

 

I absolutely agree Lakedude, but I take it one step further, correcting for odometer error. Most cars with OEM size tires will over register miles driven on the odometer, usually in the 1 to 5% range. The 2G Prius is nearly dead on and is the exception rather than the rule.

One the other end of the spectrum is my Del Sol, which with 13" OEM tires over registers mileage by 5%. But after switching to 15" tires, the odo under registers by 3.5%, which I adjust for in my tank data. Too bad the Del Sol doesn't have any way to display instantaneous or tank miles per gallon like the Prius does. I have had to make due with a vacuum gauge (simulating instantaneous MPG) and a digital fuel gauge (simulating tank MPG). Not quite as good for hypermiling as a MFD, but I guess it will have to do.

 

Interesting points, all. I guess I'll just have to chill.

My one objection is regarding the bladder and its influence on the mpg...why should that matter when I'm going off of the number of gallons that I've added? Maybe I misunderstood?

Thanks, all.

--jay

 

The bladder expands and contracts as the ambient temperature changes. As as result the tank capacity can change as much as 1.3 gal. (according to my 06 P. manual).

Consider filling up when the bladder causes the tank capacity to be at 10 gallons. Now, consider a fill-up a few weeks later when the ambient temperature causes the bladder to contract such that the tank can hold as much as 11 gallons. Instead of replacing the 10 gallons you used, you will actually add 11 gallons to the tank. This will cause the calculated MPG value to be low, X mi/11 g instead of X mi./10 g.

Hope that helps.

FW it's worth I've taken to using the display as my measure of FE. I expect it is accurate to within about +/- 5%, not bad accuracy for consumer grade electromechanical devices. Only a very well crafted/executed experiment can validate this assumption. But it is seems "about right" based on information posted by the those who've reported results over many tanks.

 

Thanks, 'lectronimo. That paints a more clear picture for me.