computer accuracy

Tank-to-tank readings can vary widely because of the Prius' flexible fuel bladder. I suggest not drawing any conclusions until after several tanks. Those of us that have tracked it over several tanks generally report close agreement between calculated MPG and what the car tells us. I've been doing so since October, and the numbers are within 2% of one another.

You may want to consider totally disregarding the first tank. It's entirely possible the dealer didn't fill it all the way.

 

Yeah, first two tanks for me were goofy. My third was where I was hoping it would be. And for me the MDF read 45.4, but I calculated 43.7. But I was only able to place 10.002 gallons back in the tank, so the 43.7 would actually have been higher if I wanted to push the tank limits.

So I am happy knowing my MPG is AT LEAST 43.73 on my third tank. I excpet it will increase as I learn to drive better and the outside temperature increases.

I am excited to get my first 500 miles in a tank! I was at 475 last time, and 490 before that. This time I think I will do it.

 

Don't try to calculate mileage based on manual calculations, unless you are willing to do it over a LARGE number of tanks. As has been pointed out over and over again, the variation from the tank bladder introduces too much uncertainty for useful numbers. The more tanks you use, the less the variation, so it is a function of how much error you are willing to live with. I recommend you use at least ten tanks for any sort of decent result. Otherwise, the MFD does a really good job with mileage calculation.

Tom

 

others have stated that this tank bladder does not actually change the volume (gas capacity) of the fuel tank. Can someone clarify what this bladder is, and how much it affects fuel tank volume?

 

It's a flexible inner shell inside the hard outer shell of the tank. It can vary greatly in volume, dependent on such things as age of the car and the ambient temperature. It's smaller in cold weather, for example.

Fortunately, that's only relevant as trivia, and not really something we need to do anything about. Just fill up when the gas gauge is low-ish, and then drive some more!

If you're meticulous about keeping records of fillups then a running average can be fairly accurate after many tanks. I've had individual tank calculations vary from the MFD by a lot, but the average of MFD readouts and miles/gallons for the lifetime of my car agree pretty nicely.

 

See pictures here:
http://priuschat.com/forums/prius-technical-discussion/30593-gas-bladder-exposed.html#post394632

See documentation here:
http://www.autoshop101.com/forms/Hybrid13.pdf

 

Sometimes my MFD is off 20% while sometimes it matches my calculations. Usually they are close. The higher differences come in colder weather. On this tank the difference was .1mpg. It's normal so relax--and the last pips go fast.

 

I have been keeping track of my mileage, odometer readings, etc. and have found a consistent 5-6% error in the MFD mileage reading. By keeping track of miles and gallons actually used, I am getting about 45.6 mpg over 11000 miles. Using the MFD to calculate gallons used, and then computing lifetime mileage, I get 48.2 mpg.

I thought the error would decrease over time. It did, but then settled down to the 5-6% error, with MFD always giving higher numbers than actual.

 

Many people report the MFD being a bit on the high side, but not as much as yours.

Tom

 

I've got the '08 Honda Civic Hybrid, most recent tank which was my fourth fill-up the computer read 45.9 but hand-calculated at 48.3 so the Honda is screwy the other way (undercalculates) but who knows which is most accurate. All I know is that I'm thrilled with the MPG's here in COLD Chicago and I will note that my neighbor's prius is only getting 39MPG hand calculated which I'm shocked to find out since he claims to be an expert hypo-miler or whatever.......I will admit though I wish I had gotten the Prius because (didn't realize this when i bought the Honda) the back seat won't fold flat!!

 

JD---I've noted in my first 4,000 miles that the MFD reading runs about 3 to 4% higher than my actual calc. math I do on each fill up... (I've learned to use the "slow stream, same pump, stop on first auto nozzle stop, and don't try to force any more in....! trick...) Other interesting note is that now that I've finally got my Scan Gage II, (and have it properly calibrated after 4 tanks--it's predicting gal. needed on fill-up to within 1 tenth gal. each time now...) that it consistently reports 1 to 2 mpg less than the MFD on individual trips... Does this perhaps support some of the other comments rel. to the MFD reading just a bit too optimistically??? 2± mpg on 45 would be about 4%±....? Have other Scan Gauge II users corroborated this? Best, Wes Shaw

 

Wes, is your 3-4% on the total for the 4,000 miles, or on a tank by tank basis? It sounds like you are doing it on each tank. The only way to get an accurate hand calculation is to do it over many tanks. Ideally you should total all the fuel used and divide that into your 4,000 miles. Your tank filling technique is irrelevant if you total over a large number of tanks. On a limited number or a single tank, it will help minimize the error, but the error term is still so large as to make the calculation effectively useless.

Tom

 

Tom... The 3-4% difference is largely something I've noticed since installing the Scan Gauge II and comparing its mpg calcs. with the MFD readings on a 'trip by trip' and/or 'day by day' basis... i.e. I'm presently tending to use the MFD mpg calc. in short term——not over the length of the tank. I reset it after a given trip to observe + or - mpg changes given weather, road surface, driving techniques, etc. . (I often make a repetitive 45 mile round trip run from Winter Harbor to Ellsworth over, initially, 8 miles of rural road filled with potholes, ice-covered, snow packed, slush laden, etc. to coastal Rt. 1 (55 mph state highway) and then 15± miles to Ellsworth and then a return...). It is proving a good test of my learning curve in p & g techniques, et al. that I'm working on courtesy of this wonderful site...

Since acquiring the Prius in late November, I've recorded in a car log the actual tank fills, etc. as I've done most of my life in all my vehicles. I've also been recording the tank fills, etc. in the Greenhybrid and CleanMPG databases and there is general agreement between the three that I'm averaging 44.7± mpg on the nasty, slushy, oft snow-covered & unplowed, icy, sand-laden, pot hole-filled roads we've lived with this present horrific winter on the Eastern Maine Coast!...

Again, the 4% difference is appearing largely when I compare a given trip's ScanGauge II mpg calc. with the Prius' MFD reported mpg for the given trip (after a reset of the MFD at trip's start). I'm tending to feel that the ScanGauge calc. is perhaps more accurate—for some reason???—than the MFD. (Again, I've spent time calibrating the ScanGauge/tank cap., etc. and this is, again, my reason for the attempt at consistency on the fill-up process...) I do realize that air temp, fuel temp, and baro. press. can affect this to a degree but I'm tending to find agreement to 0.1 gal on actual pump fill-ups with the predicted gals. indicated by the ScanGauge. Thusly, I'm beginning to feel that the ScanGauge II's reported mpg ave. is more accurate—at least on a relatively short 50± mile trip...???

Having the ScanGauge now only a month±, I'm still obviously a newbie in its use and potential but have found it a fascinating tool and 'learning window'...! Relation between Load/RPM/water temp and the MFD-reported mpg is fascinating... (Given that between the ScanGauge, MFD, and my GPS screens I'm, unfortunately, going to rear-end someone at some point in time and all my 'savings' will go down a hospital drain...! ( : >) )

One last note to those who've proven so advisedly helpful here at Prius Chat... I finally found some pipe insulation and plugged both grills yesterday and on my 45 mile r/t usual run mentioned above was happy to see an increase to 56 mpg for the run vs. the 52 I'd been repeatedly (±) getting before the grill insulation. The outside ambient air temp was perhaps 8º higher than 'usual' so that played a part, I would assume, but it might appear that the insulation & increased water & compartment temp largely gave me a 4 mpg plus???

Again, thanks for your advice... Wes