MFD vs. Calculated MPG

<div class='quotetop'>QUOTE(PriusRos @ May 13 2007, 10:21 AM) [snapback]441057[/snapback]</div>

I've seen this too:


It seems to get worse in the summer. However, recently I've been running partial tanks, I don't fill up to nozzle cut-off, as part of a gasoline energy test. But then I run to fuel exhaustion and carry a 1 gallon, spare gas can.

Running out of gas is not a recommended procedure by Toyota. So a reasonable approach might be to just run to "low fuel" warning and then add a finite amount of gasoline. It looks like there is about 1.3 gallons of usable fuel in my 2003 Prius when the alarm sounds.

GOOD LUCK!
Bob Wilson

ps. I've run out of gas eight times.

 

I just got by the MFD.

It's kinda like a girlfriend telling you how wonderful you are in bed. I don't neccesarily believe it but it makes me feel good so I roll with it!.

 

<div class='quotetop'>QUOTE(PriusRos @ May 13 2007, 04:31 PM) [snapback]441272[/snapback]</div>

I'm just modest. LMAO!!

Like I said, I'd just stick the MFD. Calculations get screwed up and are not worth the effort. Just maximize your MPG effort and call it a day. The cost difference is very minor anyway.

 

<div class='quotetop'>QUOTE(PriusRos @ May 13 2007, 08:21 AM) [snapback]441057[/snapback]</div>

I've been getting wider discrepancies too. But I trust the MFD because the Prius can measure exactly how many litres of gas it squirts through its injectors and exactly how far it has moved. So its measurement is accurate. This is good because my MFD mileage is almost always lower than my calculated mileage. Why it doesn't even out, I have no idea!

 

Actually they look right on to me, except that middle tank. Something must have been wrong there cause I've never seen the MFD be UNDER the calculated, let alone by that much. Other than that tank, your calculated MPG is about 88% of your MFD MPG for every other tank. I can't say that that in itself isn't weird though.

I was under the impression that most of the time the MFD was off by a set number, give or take. Like mine is usually off by about 2.5 MPG no matter what.

 

<div class='quotetop'>QUOTE(Fibb222 @ May 13 2007, 08:50 PM) [snapback]441318[/snapback]</div>

Actually the car only knows how many times the wheels have turned and how often & long the injectors are open. Variations in the fuel pressure control and tire diameter changes (wear or different tires) will cause the MFD number to not reflect reality. Still, the MFD number should be quite consistant and be a small fixed offset from the "real" number. Using a calibrated 5th wheel to determine the correction for the odometer and a measured quantity of fuel would be the most accurate way of determining MPG. That is what the better car tests have traditionally done. Consumer reports used to do it this way (and probably still does).

- Tom

 

Over 2½ years, a 2004 Prius recently traded for my 2007 Prius, I have consistently found the MFD to be less than 2% higher from hand-calculated MPG on average - in my opinion "Close enough for 'Government Work!'"

 

<div class='quotetop'>QUOTE(PriusRos @ May 15 2007, 05:31 AM) [snapback]442320[/snapback]</div>

I don't know what to say except that over 15 fillups, the vast majority has the calculated mileage higher (and in my case that's bad as I am using metric L/100km) than the MFD mileage. The difference can be about 10-13% in some cases. I don't know why this is, I can't explain it. I was hoping I would have some calculations balance out the other way but so far I don't. When I fill up I don't top up. The first time the pump stops pumping - I'm done. Not that that should matter.

Sure the MFD measurement isn't perfect but it given the variable size of our gas tank bladders I'd rather go with it than the calculated mileage.

 

After 41 tanks, my display MFD is higher than calculated by only 1.37 mpg.

I have had MFD higher by six mpg and I have had calculated higher by six mpg.

Ten times calculated MPG has been higher and 31 times MFD higher.

So I would say overall calculated almost same as MFD.

 

The MPG per fill-up is good to know, but I think a more meaningful number is the
actual lifetime MPG average, i.e. total gallons / total miles driven (from cradle to grave).

I have a spreadsheet that tracks both MFD numbers and Estimated values, as well as
outside temperature at fill-up time.

I have a related question: after a fill-up the "Miles" number is reset to zero,
but the MPG number is not reset. Any idea why?

Thanks!

(07 Mag Grey #3)

 

I agree that the best way to measure lifetime MPG is total mi/total fuel.
That is why I also use a database such as the one provided by Green Hybrid [along with graph of the data].

Usually MPG just after fillup goes down from 99.9 but sometimes it may start around 40 mpg and move up.
Besides total ODO miles, you also have trip A and trip B to use for longer calculations.

My lifetime Prius data [for three years] is 21595 mi/341.691gallons equals 63.2 mpg.

 

Interesting - just took a 2360 mile round trip vacation from NW Ohio to New Orleans. Reset my MFD prior to starting and left with a full tank. Lots of hills and A/C usage especially in the south. Wife used cruise most of time - I didn't. At the end of the trip my calculated mileage was 45.8 mpg and my display said 46.9 mpg. Only about a 2% difference over the long haul.

(Have Goodyear Comfortred tires - 40/38 psi front/rear)

 

<div class='quotetop'>QUOTE(Fibb222 @ May 15 2007, 01:07 PM) [snapback]442569[/snapback]</div>

This is waay off topic, but do they still talk about "mileage" in Canada and Europe? Shouldn't it be "kilometerage"?

I agree with the posters who say look at lifetime miles divided by lifetime gallons. Bladder doesn't affect that. Only problems there could be inaccurate pumps, but at that point its out of all of our hands...

 

Both fillups on 5/7 and 5/11 were for 6.90 gallons?

Have you double checked that this is correct, or did you do that on purpose at the pump?

Just a thought...

 

I'd just like to get above 50MPG finally...

My sorry nice person is still stuck below 40.