I know that the calculated mpg (miles divided by gallons on fillup) is not very accurate because of variations in pumps and the fuel bladder, etc. but it seems that recently I've been getting wider and wider discrepancies. I try to fill up the same way each time -- ie, after it first shuts off, I then squeeze it for one more click. In the past, the difference between the MFD mileage and the calculated mileage has not been that much -- usually between 1 and 3 mpg difference, but on my last few fillups it's been showing large differences. As you see from the chart below (sorry, I can't figure out how to space the columns properly), the calculated mpg is significantly lower than the mpg on my display. The 5/7 discrepancy could be explained by the previous fillup, where the calculated mpg is higher than the MFD (maybe I didn't quite fill the tank on 4/22, causing me to have to pump more gas in the 5/7 fillup). But then why would my 5/11 fillup be so off? I'm particularly upset by this last reading because I thought I was finally getting over 50mpg again on this tank after months of being in the low 40s and even 30s due to short trips and cold temperatures! Code:           Trip                    MFD        Calculated Date      Miles    Gallons      MPG            MPG 3/03/07     235      7.93        33.60        29.63 3/28/07     312      9.29        35.90        33.58 4/22/07     373      8.39        38.50        44.46 5/07/07     278      6.90        45.80        40.29 5/11/07     306      6.90        50.60        44.35 PriusRos: I put your numbers into a "code" block in hopes of making it easier to read. Hope you don't mind. -Tony
<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(F8L @ May 13 2007, 12:50 PM) [snapback]441093[/snapback]</div> Of course! Why wouldn't you? :lol: OTOH, how much do you choose to believe when she isn't that impressed? Hey, what's this got to do with MFD MPG anyway?!
<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!
<div class='quotetop'>QUOTE(PriusRos @ May 13 2007, 11:21 AM) [snapback]441057[/snapback]</div> Thank you -- of course I don't mind. But please tell me how you get the spaces between the columns.
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(Alexstarfire @ May 15 2007, 01:08 AM) [snapback]442229[/snapback]</div> It's fairly easy to for the calculated mpg to be more than the MFD -- if, for instance, you don't completely fill up. The next fillup would then usually be the other way around. Which would explain my 4/22 and 5/7 readings. Up until recently my calculated mileage would be fairly close to the MFD mileage, varying by no more than plus or minus 3 mpg and usually much closer. The variances should cancel themselves out over time. In other words, the calculated mpg should not be consistently less than the MFD mpg. There is something wrong if your calculated mpg is always less than the MFD. Although the Prius computer might give you a more accurate current reading, over time, it should be the calculated mpg that is closer to the actual. I've been keeping a running record of the amount of gas I use for each fillup and the odometer reading. One can't argue with total number of miles driven divided by total amount of fuel used.
<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.
I guess what it boils down to is that if you want to go with the MFD reading, that's okay. But when you think about it, after 10,000 miles and x gallons of gas, 10,000 divided by x is going to be your ACTUAL mpg, regardless of what the MFD says.
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...
