Only 47mpg on the computer but 52 at the pump?

[uart]

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?

[PaulHS]

Originally Posted by uart

Is it possible to have such a large discrepancy between the computer displayed MPG and the actual MPG as calculated at the pump?

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.

[Bob64]

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!

[uart]

Originally Posted by Bob64

Search for 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.

[boppo]

Originally Posted by uart

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.

you could get a gallon of gas and bring it with you and run it dry and see what you get.

[ceric]

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.

[qbee42]

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

[Shawn Clark]

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.

[qbee42]

I missed that the OP is from Oz. Scratch my bladder comments since you don't have one.

Tom

[uart]

Originally Posted by qbee42

I missed that the OP is from Oz. Scratch my bladder comments since you don't have one.

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.