Is it possible to have such a large discrepancy between the computer displayed MPG and the actual MPG as calculated at the pump? Ok it might be a bit premature to ask this question because I haven't actually filled up yet to calculate it for sure, so maybe it's just wishful thinking but here goes... Since the last fill the computer display has shown an average fuel economy of only 47 MPG. However I seem to have travelled way too many miles for that to be possible. It's currently at 535 miles with 3 pips still left on the gauge! Last time I filled up the gauge was similar and it only took ten gallons. I wont know for sure until I fill up tomorrow, but I really don't think it's going to take more than 10.5 Gallons maximum (unless the gauge is really truly bad). So I'm definitely starting to think that the actual MPG is going to be more like 51 or 52 rather than the 47 that's currently showing. Has anyone else ever had such a big discrepancy like this?
Actually that's not such a large discrepancy. Typically, however, the display reads high when compared to the pump reading. My Prius has had pump readings as much as 21 mpg lower than the display, but I admit the car's tank fillups are way on the quirky side.
Search for Gas Bladder. Basically the 2nd gen prius does not have a gas tank, but rather a gas balloon. You will never know exactly how much gas is left. The presence of this gas bladder has impacted the fuel gauge, which is nicknamed the guess gauge. Anyway, your MPG calculations are off because they are based on the false assumption that you have gone XXX miles on the Y gallons you managed to stuff in the gas bladder. The Y Gallons you just put in does NOT EQUAL the gallons actually used for the past XXX miles!
Yes I know what you're saying because I had been thinking the same thing. But now the miles are just so high that it's starting to look more and more unlikely that the computer could possibly be right. The tank is only 12 gallons and at the rate I'm going I should have used 11.4 already. So how could three pips on the gauge be only just over half a gallon, it seems impossible. Anyway one thing for sure is that something doesn't tally, so I better fill up tomorrow before I run out. Either way I'll know for sure after the fill up and I'll post back here with the results.
Unless you use the same pump at the same gas station to fill up at both times, the click-off method is not as accurate as you might think. If you do it over time, the error will average out and becomes accurate. The short answer is that pumps click off at different points due to different calibration of sensors.
You really need to do some reading on this forum. I'm not going to spell it all out again, but in a nutshell here it is: 1) Your Gen II Prius does not have a 12 gallon gas tank. It has an 11.9 gallon gross capacity tank with an internal bladder. This reduces the usable capacity to something like 10 to 11 gallons. 2) The bladder creates a large uncertainty with each fill. Because of this there is no way to do accurate hand mileage calculations on a small number of tanks, let along a single tank. You need to total a large number of fills to reduce the uncertainty to a usable level. 3) The MFD mileage on a Gen II is fairly accurate. Most owners find it is around 2-5% optimistic, based on long term manual records. 4) Never assume you know how much fuel remains in Gen II fuel tank. Do you know what we call people that do that? Pedestrians. Tom
Do some reading of the forum. This is actually a small discrepancy...I've had 114.5 mpg calculated and 53.8 mpg indicated, thanks to the bladder/vapor recovery system preventing a good fill on a half tank. In Australia you don't have the bladder IIRC but with a small tank you should still expect considerable tank-to-tank variation (percentage wise.) Not sure whether your vapor recovery system is finicky or not over there. My accumulated MFD to calculated variation stands at +0.76% (indicated vs. calculated) over 30 tanks.
That's ok Tom, it would be pretty easy to miss since I was using miles gallons and "gas" (we call it "petrol" here). Anyway I had all may data in Litres and Km but then I decided to convert everything to miles and gallons so most readers would follow it easier. BTW it's nice to know we don't have the rubber bladder thingy here, it sounds like it might be trouble for not much benefit.
The 'benefit' is to the environment, it reduces vapors and aids vapor recovery. For the owner, not so much.
Yeah I figured that's what it would be for. I thought that most conventional tanks had pretty good vapour recovery systems these days anyway.
Ok I've filled up and the numbers pretty much blow away the prevailing argument that this discrepancy is all due to inconsistency of the fill volume. Now to be totally sure I drove it until the gauge was down to just one pip (but still a solid pip - not yet flashing). The distance was an amazing 1011km (628.2 miles). Remember that the MFD displayed fuel consumption was 4.9L per 100k (48MPG) so that means it should have used almost 50 Litres (13.1 Gallons). Absolutely impossible right, since that's over a gallon more than the tank even holds. So I was right all along, this is a very genuine case of a large discrepancy and not just a case of error on my part. So I filled up at one solid pip on the gauge and I filled in nice and slow to be certain that the pump didn't cut out too early. I filled it as much as possible (remember I don’t have the bladder) and all I could get in was 41.9 Litres (11.1 gallons). So amazingly while the MFD was showing a calculated consumption of 4.9L/100k (48 MPG) it turns out that my actual consumption was no worse than 41.9/10.11 which is 4.15 L/100k (almost 57 MPG). Wow this is much more like what I was aiming for. Though I'm much happier with this figure I'm still a little miffed as to why the MFD calculated value is so far out.
Do you have factory rims and wheels on your car? Perhaps you've installed larger/smaller tires that are throwing off your calculations?
That's pretty odd. Tire diameter comes to mind but this doesn't explain the MFD vs. tank difference. The odometer is used for the tank calculation and should be the same as the distance travelled basis for the MFD. Have you actually checked the odometer versus known distances? At least then you would know if the odometer was correct.
Good questions about the odo accuracy and rim/tire diameters. I don't have a GPS but I was going to compare the odo reading against another car this weekend. I'll post back with the results. I just bought the car second-hand so I don't really know the full history, but the tyres are correct as listed in the specification and the rims look like standard. Over the last few hundred km I've been checking th MFD distance display against that on the odometer and they're identical (as expected of course - it wouldn't make any sense for them to use two seperate distance measurements ). So I don't really see how the odo reading can be a factor here (even if it is slightly off) because I'm using the very same figures to calculate my fuel consumption. It seems to me that the only variable remaining is whatever measures the fuel usage. Perhaps the sensor that meters the flow is out of calibration and the computer thinks that it's squirting a bit more fuel than it actually is?
