Mileage vs pump calculating

[qbee42]

The standard most often used for retail gas pumps is a maximum error of one cubic inch plus one cubic inch per indicated gallon, but never less than two cubic inches. For five gallons, this means an allowable error of +/- six cubic inches, which works out almost exactly to your +/- 0.5%.

Tom

[micheal]

I would concur with the other sentiments of MFD best one tank measure, calculated best long term measure.

Something I have noticed because of the combination of the variable bladder and temperature volatility is that calculated tanks can be very disparate with the MFD on an individual tanks. I have had several occasions when I would fill-up in a cold snap and by the time the next fill-up was needed, the temperature would be 20-30 degrees warmer. Nevertheless the calculated mpg would be extremely low. The opposite has occurred with warm spells then filling up in a cold snap.

[dwreed3rd]

Originally Posted by carz89

Regarding accuracies, gas pumps in most states are certified to be accurate to within 0.5%. Although a small fraction of gas pumps will fail their certification test, they inevitably get re-calibrated and then pass inspection. Their accuracy drift over time is probably small, as I imagine that most pumps pass inspection year after year without recalibration.

In my Prius, I have the original tires, inflated to 42 psi. In comparing my car's odometer and speed displays to my Garmin GPS device while at highway speeds, I've found that my car exaggerates distance and speed by 2%. (I'm assuming my GPS is dead-on over reasonable distances).

If you want to be as accurate as possible, you should apply a scaling factor for your car's odometer by determing the ratio of your car's displayed speed divided by the GPS-indicated speed (best to do this on a level highway using cruise control to minimize speed fluctuations).

long term -- go with your odometer (scaled) divided by cumulative gallons pumped at the gas station.

short term -- go with your MFD miles-per-gallon, applying the same scaling factor.

Hi Cars89: Two things. If you read my earlier post, you will see that dividing miles driven since last fillup, by gallons pumped, this fillup, is meaningless unless the tank volume is a constant. Because of the bladder in the Prius' tank, the gallons pumped at this fillup is not necessarily equal to the gallon used to cover the miles driven. In would be like trying to track your gas mileage on a conventional car but never filling your tank to the same level from one fillup to the next. Manual calculations on a conventional car works as long as we fill the tank. Because we assume it is replacing the fuel we used, the amount that we pump should be ~equal to the amount was used since last fillup. Not necessarily the case because of the bladder in the Prius' tank.
Second: Your MDF mileage may actually be the more accurate. I'm assuming the GPS usually shows less mileage than the MFD. This may vary by GPS brand and sofistication of the software. It's inherit to GPS since GPS is basically 2 dimentional and tracks your mileage by plotting the distances between a series of points. It does not necessarily take in the terrain. Just as your mileage can vary on a windy road between two points from that of the distant as the crow flies. There's even a greater diffence ove hilly terrain. My assumption is that your GPS would be dead on with your MFD driving across I-40 in Oklahoma, as long as you didn't drive around for 10 minutes in the parking lot to keep the toddler in the back from waking up, while your spouse ran in to get something at the grocery store; while there would be quite a discrepency driving on the Blue Ridge Parkway through Virginia.
The first I'm sure of, and yes, averaging it over number of fillups would average it out, but any single fillup could be way off thr average. The second is only speculation. Hope this helps.

[DaveinOlyWA]

we can argue accuracy of one over the other until the cows come home and it will do no good since both measure different things.

1) the MFD tells how how far we drove on said amount of gas...period...if straight miles per gallons is all you need, then this is it.

2) the pump tells the volume of liquid was put into the tank. why is this different? ignoring measuring inaccuracies of the pump, we also have to consider contamination (albeit, very low we hope but hardly nonexistent) and evaporation. sure the bladder helps. but it does not eliminate the possibility.

i think we can all agree that the pump gallon calculation will be higher (or mpg will be lower...) than the MFD number... not by a lot...less than 2% for me anyways...but that is what it will always be.

and one parting comment about the claim that "manual calculations always work if the tank is filled to the same level".... i say it works because there is usually nothing else to dispute the results.

[dwreed3rd]

Originally Posted by DaveinOlyWA

we can argue accuracy of one over the other until the cows come home and it will do no good since both measure different things.

1) the MFD tells how how far we drove on said amount of gas...period...if straight miles per gallons is all you need, then this is it.

2) the pump tells the volume of liquid was put into the tank. why is this different? ignoring measuring inaccuracies of the pump, we also have to consider contamination (albeit, very low we hope but hardly nonexistent) and evaporation. sure the bladder helps. but it does not eliminate the possibility.

i think we can all agree that the pump gallon calculation will be higher (or mpg will be lower...) than the MFD number... not by a lot...less than 2% for me anyways...but that is what it will always be.

and one parting comment about the claim that "manual calculations always work if the tank is filled to the same level".... i say it works because there is usually nothing else to dispute the results.

daveinolywa: I'm not arguing accuracy. I'm trying to make the point that manual calculations using gallons pumped and miles driven is O.K. for a running mpg average, probably over a period of 10 or more fillups. Any manual calculation for a single fillup using gallons pumped may not be representative of the actual mpg between fillups. When I say I got x mpg on the last tank, I mean "miles driven divided by gallons used" since last fillup. This works for a conventional car because the tank capacity is a constant. When I refill the tank the gallons pumped shoud be, for all practical purposes, ~equal to the gallons used, and can be used for a manual calculation. With the prius, however, because of the bladder, although it's an 11.9 gal capacity tank, you may have filled it to the point that it had 11.0 gallons full. You drive 420 miles. You fill up the tank. It stops at 7 gallons. 420g/7m= 60mpg. excellant mileage. The problem is due to the bladder, this time the tank only contains 10 gallons. You actually used 8 gallons on the last tank not the 7 gallons that you replaced. 420m/8g = 52.5mpg not 60. The error in using gallons pumped, is that gallons replaced may not be representative of the gallons used since the last tank. It will average out over a number of consecutive refills, but is basically meanless for any individual tank refill. It really took me awhile for it to stink in. I really think this is were some of the other members are getting confused. We were so used to replaced gallons always being ~equal to gallons used from previous tank, but they are not for the Prius.
At least, that's how I understand it and that's all I was trying to point out.

[Old Bald Guy]

Originally Posted by My08

The display says (ie:51.7 as the average). I always fill up. When I calculate the amount of gas pumped to the miles driven, this most times doesn't come close to the display (avg)

Your thoughts ?

I have kept log books on cars for over 40 years and my father before me. A couple of things we learned ...

1. Speedometers are never totally accurate. Easy way to check your odometer ... get on a nice long stretch of highway and check your odometer agains the mile markers for at least 10 miles ... 20-25 is better. I have routinely done this for years all over the US. Usually I am only off about 1 or 2 tenths in 10 miles, but I have had some cars WAY off.

2. No two gas pumps will kick off at the same point of filling your tank. The shutoff is controlled by air pressure and THAT can be affected by temperature, how fast you are filling, how the nozzle is in the tank. etc.

Keep in mind ... THERE IS NO SUCH MEASUREMENT AS MILES PER TANK ... I emphaises that for the people who are stuck on that number. Miles per gallon has been a standard for over 100 years. Your MPG per tank will not always be the same ... but over many tanks ... it will get closer and closer to being accurate.

Also, on the Prius ... or most cars built in the past 20 or 30 years ... DO NOT FILL PAST THE SHUTOFF ... your pollution equipment requires that headroom built into your tank to work right. Unless, of course, you are trying to damage your pollution equipment.

[dwreed3rd]

BTW I want to stand corrected on a misconception that I had. Looking at the illustrations in the Bentley manual, I misinturpreted the Fuel filler pipe and the Air vent's entry into the tank. I thought that the bladder was for the air space and the gasoline went into the tank. I had it backwards. The gasoline does go into the bladder, and the air space is the area surrounding the bladder inside of the tank. This post is not for those members that already understood this, except for an apology for my ignorance, but for those members that did not know and that I may have misled. It's scary though. I think I'm beginning to understand how it works and what is behind some of the fueling issues.