2008 Gas Mileage: MFD vs. Purchased

Because there is only one measure of distance in either the "purchased" calculation or the MFD average, i.e. the odometer, the observed difference are due to the measurement accuracy of the fuel consumed (tire pressure and tread thickness will not add a bias in this particular comparison).
I, too, have observed the same 1.5/52 or 2.9% bias over the first two years of my 2007.
It boggles my mind that the fuel injector (almost counting drops) is that close the gauge at the pump. I have always wondered how accurate the pump gauges are. The fact that we are all observing the bias in the same sense (the MFD reading higher than the "purchase" calculation) seems to imply that it's systematic.
Toyota might have an interest in displaying "rosier" numbers on the MFD, but the gas-stations have a monetary interest in charging you for more fuel than you actually purchased.
So my question is, does anybody in this group know what the measurement tolerance is for the absolute calibration of the fuel pumps? Could it be of order a few percent? And could it be that the gas-stations are milking every last penny out of that tolerance?

 

JesjesRick,

One thing that is important to do before drawing such a conclusion is to measure several known distances to check the odometer (the longer the better, do it with dry roads.) I've done this with other vehicles and noticed that the odometer errors some time match another tire spec option for the vehicle. Haven't done it for the Prius yet.

 

The base tyre option is 185/65 R15. The North America Touring and UK spec is 195/55 R16. The touring tyre works out to be 0.1% smaller (in circumference) than the base tyre.

If there is an issue, it's not down to the tyres, assuming the owner keeps to the OE sizes.

 

And before someone asks: tire circumference is a very weak function of tire pressure until the pressure is very low, well below the recommended pressure (at which point sidewall flexure is killing MPGs anyway). The radial plies inside the tire restrain the shape very strongly.

 

Jesjes;

blaming the pumps at the station is probably the way to go. in WA State a TV station, based on several write in complaints, hired an independent company to go around and calibrate several gas stations in the Seattle area. 1 in 9 had inaccurate pumps greater than the allowable error level which is like .75% of 5 gallons. (not sure of the percentage error number but was used against a calibrated 5 gallon container which has marking based on temps)

of the inaccurate pumps, better than 4 in 5 were inaccurate in favor of the gas station. now it sounds like something fishy is going on, but upon further investigation, here in WA State, all calibrations are recorded and logged to check for tampering so its simply not likely that the stations tampered with the pumps.

there is however a part used in the metering process that becomes inaccurate as it wears requiring it to be calibrated and that state mandates each pump be calibrated every 3 years. it was also noted that 1/3rd were out of date (when i saw this, i went around and checked the calibration date for pumps in my area and had nearly half were out of date including some that were nearly 5 years old!!)

KING 5 went to the state board of weights and measurements to ask about the calibration process and they agreed that they were way behind, but simply did not have the money to hire more people (during this time, summer 2005, WA State was enjoying a 2 billion per year surplus) and that there were only 7 crews of two responsible for doing every single gas pump in the entire state of WA. they did however state that they field dozens of complaint regarding suspected pumps and that about ¼ of the pump inspection were unscheduled making the teams less effective since it would require a special out of the area they were working

well, all this got me to thinking. with the bladder, its tough to determine accurate fillups, but i did notice that there was a station in West Oly where i did seem to get lower numbers. so i tried the station 3 times in a row and each time i was 1 to 2 gallons low as compared to the MFD. (these pumps were out of date by just over 4 years) well, i wrote a letter, stating info i had. and waited... and waited... and waited.

my letter dated Aug 7th, 2005 was initially answered aug 20th acknowledging my complaint. then i received another letter in FEBURARY 2006 that advsd me that they had detected inaccuracies that "were not at a level that warranted further action other than recalibration"... so iow, it was subtle statement telling me that sueing for retribution for gas paid for but not received would not be an option....

sure enough a visit to the station (no i did not buy any gas!!) did have inspection stickers dated Feb 2006 on it (obtw, last time i checked the station which was a few months ago, they had been recalibrated after only just over 2 years they now at july 2008... now whether it has anything to do with the fact that its the busiest station in town and also the one closest to the Thurston County courthouse is anyone's guess

 

Odometer accuracy is canceled out in this experiment. The same odometer readings are used for both the MFD and the manual calculations.

Tom

 

Yep, I realized that after I posted. Wondered how long it would be until someone mentioned it. eep:

 

You could say that you were just testing us.

Tom

 

Isn't that what JesjesRick said in post 21?

 

Thanks DaveinOlyWA for recounting your observations and experiences at the pump in WA. I am not really a conspiracy theorist and posted half out of jest. And even though 2 to 3 % are small numbers for many real world measurements if they reported consistently by everybody it becomes a measurement bias and worth investigating further. It would be easy enough to check without the bladder problem, if you had a calibrated 5 gallon container. 2.5% would mean a fill of 5 gallons at the pump would ALWAYS be short 1 pint. Where the conspiracy theory comes in is that if every Prius owner reports essentially the same bias (let's say 1 to 3% from what I have seen here) then at least all the gas-stations that Prius owners fill up at would have to be cheating their customers.

The alternatives would be that the bias is in the measurement of the fuel injected into the engine.

I don't think there is a third alternative that both measurements are accurate, because that would imply that 1 quart per tank would be lost between the time it is filled and the time it is burned. Granted gasoline is pretty volatile, but I thought this whole bladder system was designed to reduce the amount of evaporation. And if it was due to evaporation then our friends in the always sunny climes would have to be reporting the biggest discrepancies, right?

And yes, I did try to say that since the distance driven is measured the same in both calculations it can't be the source of the bias.

 

Or a selection effect plus an "invisible hand" conspiracy:
- Selection effect: people who calculate MPGs manually and see it smaller than the MFD-reported MPGs are more motivated to post about it than are people who don't do it, or find no difference, or see better MPGs on the MFD.
- "invisible hand" conspiracy: gasoline retailers have a strong profit motive to over-report the amount on the pump, and (speculating here) 2% is about the maximum that they can exaggerate before arousing suspicion in casual customers and/or getting slapped with big penalties by their state's weights-and-measures bureau. No collusion required.

 

FYI, I found the official tolerance values that I was looking for here:
ts.nist.gov/WeightsAndMeasures/Publications/upload/3-30-09-HB44-FINAL.pdf
See table T.2.
Petroleum Fuel Pump acceptance tolerance is 0.3 % and maintenance tolerance is 0.5%.

 

And the speed limit on Arapaho Drive is 40 MPH :_> By which I mean, law and practice can be different things.

 

I might have a solution to the dilemna over indicated MPG vs. calculated: ambient temperature. If the gasoline is cold in the tank, then it will be cold in the injector. Cold = slightly less volume. The underground tank temperature is not varying as much as the cars tank temperature so the MFD will calculate X gallons used, even though more mass was injected. The gallon metered from the pump shrank in the tank during cold weather...

Why do I say this? Well, I've noticed a shift in the bias of the numbers in the last three fills...all quite negative (-9.1%, -6.7, -7.1) while prior tanks in warmer weather (on average) had shown a more presbyterian distribution with no effective bias. This is highly improbable from a statistical perspective since the cumulative error is large for these consecutive tanks. Now perhaps the bladder is stretching or the fuel gauge has drifted, but it has caught my attention.

Someone with reams of good temperature data (Tony?) for his fills might be able to detect an error that correlated with temperature. I might be able to approximate it by extracting local weather data, but don't have enough points to make a conclusion.

 

i dont know if gas is different than water, but its not like metal, iow, the colder it is, the smaller it gets.

ice floats.... ALL TYPES of ICE (or just about all anyway) which means that its lighter. but like most liquids, i am guessing the density difference is small enough to not consider

 

Water is a very strange and unique substance. It gets denser as it gets colder, but then expands as it goes through the phase change to solid. Water also has a very high specific heat and can dissolve almost anything.

Tom

 

Almost nothing behaves like water. There are some peculiarities of its bonds both in the liquid and crystaline forms. Few substances are more dense as liquids than as solids. Then there are the other properties like the density inversion in the liquid phase, specific heat, etc.

It's not just the specific heat, water has a very high heat of vaporization and that is very handy to us. As my dad used to say, "If you gave an engineer the most exotic new way of producing energy the first thing he would do with it is boil water, same as they always have."

Hunting around I found the volume expansion coefficient for gasoline is 0.00095/C. So a 20 C (36 F) difference in temperature is 1.9%. Average temperature above ground around here has been 30 F for the past two months. Temperature in the ground tanks is probably closer to 60 F--although a fresh tanker load of gasoline will be quite cold during winter and it will be some time before it warms.

The MFD calc is likely done on a specific gasoline density assumption for the injectors--perhaps an annual average temp at some geographic point, or some departure from it based on the local temp at the injector (hotter in the engine bay than that in the tank.)

Of course what matters is how the injector responds to the different fluid temperatures. It might even be engineered in such a way that the thermal expansion/ contraction of its materials of construction somewhat compensate for gasoline's changing density. I've not worked with injectors before so I'm not sure how their calibration responds to temperature variation.

 

I have a 2005 Prius and was consistenly getting between 46 to 47 mpg. This has suddenly dropped to about 32 mpg. Can someone give any causes for this?

Virgildlc

 

virgil,

It is probably best to post such a question in a new thread since this is a different topic. But before you do, I suggest reading a few of the many threads with similar questions. That way, when you post you can answer many of the questions before they are asked (in your initial post) or be ready to gather the information to answer them. With luck, one of the comments in another thread will give you a head start on the troubleshooting.

Good luck and welcome!