How does MFD calculate fuel economy

I suspect it is not as complex as you think. It's probably a computer algorithm based on engine RPM. Because I don't find it to be that accurate; in fact the last tank the MFD optomistically gave me an average MPG of 45.2. When I did the calculation it was actually 42.96MPG (406 miles on 9.449 gallons of gas), which is plus or minus 5%.

 

I'd have to check again, but I think New Car Features states it is the on time of the #1 injector, assuming flow rate of that injector.

MFD may be adjusting its estimation of injector flow rate based on gas tank, as my errors seem a bit less than when I first got the car.

 

I read on forums that it takes the #1 injector timing and calculates fuel flow.

Dont put too much faith in the pump reading, either. Stations often cheat, and a certain error is allowable even at an honest station. I avoid a station I know for a measured fact to be inaccurate, always on the high side (giving me lower calculated mpg) by about 5%.

 

there was a blurb on the local news in seattle over an investigation into the accuracy of fuel pumps in the area here. studies found a significant number to be inaccurate with a 4 to 1 ratio favoring the gas station.

law "requires" that pumps be calibrated every 3 years by the state. HOWEVER, there is only 9 people assigned state wide to do so. based on my records, i filed a complaint to have one pump checked because it shorted me 3 straight times. that was last august...still waiting to hear from them. i checked the pump in jan or so and it still hasnt been inspected.

obtw... most recent inspection sticker i have seen... June 2000.

so moral of this story. you may find that once you get used to your Prius, your mileage may not vary as much as the amount of gas you pay for.

 

I hadn't imagined that this could be a factor. How did you determine that the pump was shorting you?

 

prior to using that pump, i would alternate between being over or short of the MFD reading which was caused by variances in the fullness of each tank.

i also log what pump and station i fill at. well i noticed that one fillup seemed to give lower stats once. so i tested the pump by going there 3 straight times. it averaged being low by 3 mpg. that equals my largest margin of error and the only time that i have been under the MFD mileage 3 times in a row (this is also the cheapest gas station in town which was the reason i went there in the first place)

according to the report on the news, 1 in 6 pumps statewide are inaccurate due to a known problem. a gear in the metering valve wears out due to normal usage which is why the pumps need to be calibrated. what alarms me is the high ratio in favor of the station. supposedly, the electronic pumps are tamper proof since every adjustment is automatically time-stamped and must jive with the sticker on the pump. (its a 25,000 fine to alter or deface that sticker although, ive seem a few that were so faded you could barely read them)

 

I also complained because I saw no sticker on newly installed pumps at a station I use. I was told that new pumps are not checked. They are "calibrated" by the technician who installs the pump and the state does not check these for years. They assume the installers are honest. I see variation between 2 of the pumps at that station. I no longer have much trust that the pumps are accurate and clearly not down to the third dismal that the pump reports. Things may be better in other states but in Washington you are on your own.

 

I thought that we also had to be careful making "exact" fuel calculations using gas pumped in to the car, as the change in fuel baldder size since the last tank might mean that more or less gas was pumped in than was actually used.

 

i have mentioned the lack of an inspection sticker at this one place near Ft. Lewis but they say that customers vandalize it (it did look it was damaged by keys) and they are forbidden by law to fix it.

 

LA County required inspections every 6 months. My local station must be "friendly" terms with the inspector, since after my complaint the sticker was magically re-installed.

According to the pump, I put 17 gallons into a 16 gallon tank. I was not out of gas, so the fuel pump and lines were full (which can amount to almost 1/2 gallon if dry). 1/16 gallons is about 5%.