Incorrect MPG either mfd or bladder tank

The mfd is usually pretty darn accurate, mine is about 1.5% too optomistic after 45000 miles. What isn't linear is the pips for the gas guage. They will start disappearing more quickly as the tank get lower.

 

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[imglink]http://i11.photobucket.com/albums/a199/Bobc64/prius/priusbladder.gif[/imglink]

Make sense now? The gas tank isn't a gas tank, its a gas BALLOON (bladder). Thats why the top pip seems to last forever, and the bottom pip goes so fast.

 

The MFD is generally found to be much more accurate than anything else. After taking a lot of tanks and averaging them together, people almost always find the MFD is right on, maybe 1% high. Any given tank may be off by 10% or more in either direction based on the pencil and paper method.

A lot of people like to bag on the accuracy of the gas gauge, which I think is unfair. Its pretty accurate, probably as good as any regular car, but what its trying to do is more complicated. Most people find they routinely get about 9 gallons into a tank on a fill up if you fill when the last pip just starts to blink.. Sometimes 8, sometimes 10, occasionaly 7.5, occasionally 10.5, but usually averaging out to about 9. Part of the shortfall is because there is still some fuel in the tank, and part is due to the bladder taking up space. Part of the variability has to do with the gas pumps, part has to do with the Prius' sealed vapor system, and part has to do with the bladder changing size and flexibility with temperature.

Since the tank size is basically variable, the gas gauge has a tough job to do, trying to determine what exactly full is. It doesn't want to tell you its not full when it really is, as then you might try to overfill it which can do very bad things to the vapor recovery system and spill fuel all over the place. So it dynamically tries to estimate when the tank is full based on a number of sensors. This means that on top of the normal pump stop variation, and the fact that the Prius may make this a little worse due to its more closed vapor system, you have the computer adding a bit more uncertainty onto the top. The upshot is that 1 pip can not be equated to any constant amount of volume of fuel, or number of miles. The gas gauge is generally vary reliable. When it tells you its almost empty and you need to add fuel, its almost always right. Where a lot of people get into trouble is when they second guess the gauge, by assuming they know how much gas is in the tank. Especially if they actually believe the tank holds 11.9 gallons.

In general if you just don't worry about it you'll be fine. If you live in a city with a gas station on every corner, fill up when the last pip blinks like I do. If you live in the burbs, and gas is generally pretty available, fill up sometime during the last pip. If you live out of town, maybe fill up when you get to two pips. Just pick a safe buffer for your circumstances, and you'll be fine. There is really no value to squeezing 500, or 600 miles out of a tank beyond bragging rights. Not worth the risk to the car IMHO.

There are of course times when you need to have a rough idea how much fuel you have, and how far you can go. In those circumstances here's what I do. I assume that however much fuel I put in last is all the fuel that I have. I divide that by 11 (I add one pip as the gauge often seems to pin off the top for about 1 pip) to get the rough current gallons per pip. I then multiply pips remaining, by gallons per pip, by expected mpgs for the rest of the trip. Note this may not be the same as your average mpgs. This gives a rough idea of miles remaining, and should be on the safe side. I'd still play it a little conservative though. Especially if I didn't fill at the blinking pip and am trying to project beyond the number of pips I filled at.

Here's an example. Lets say I last filled up with 8 gallons right at the blinking pip. Divide by 11 is 0.727 gallons per pip. I have 5 pips remaining, and expect to average 45mpg (fast highway with some uphill). So 5pips*0.727 gallons per pip = 3.64 gallons * 45mpg = 163.6 miles. In this case that ends up being about 32.72 mile per pip. That would actually even be lower than your 140 miles in 4 pips or 35 miles per pip. It can be much better than this with a bigger fill and higher mpgs, but it does show your case to be very feasible. I also wouldn't be surprised if a dealer fill is on the light side, and if they didn't top up at 36 miles. If often takes 60 miles or more to drop the first pip.

I think this is where people run into problems. They think in round numbers and say, ok the tank is about 12 gallons, and there are 10 pips, so 1.2 gallons per pip. At 50 mpg I should get 60 miles per pip! We've already shown above it can be half that under some conditions. Even if you know about the bladder and say 10 gallons, that would be 50 miles per pip. Either assumption will get you into trouble sooner or later.

For the most part you can just ignore all that. If you just drive the car and put fuel in when it says "Add Fuel" all will be well.

Rob

 

you will get about 390 to 410 miles from that tank (assuming your average stays the same). Normally a full tank carries about 9 gallons.

 

The gross capacity of the N.A. Prius fuel tank is 11.9 gallons. The usable capacity is less. At most the usable capacity is somewhere between 10 and 11 gallons. I always figure 10 gallons as it is safer.

As the other posters have noted, you never really know how much gas you have until you don't have any. Out of gas is the only sure state. When the gas gauge tells you to get gas, get gas. That's all you need to know.

Tom

 

I would assume the opposite. I would always assume the dealer will almost certainly never completely fill the tank and only fill high enough to move the gauge to show full.

 

you will always get the worse mileage and miles per tank on the dealer tank, wait till you fill up the first time and see how things go.

 

I agree that its on the low side, but not impossible. You said you had gone 140 miles and had 6 pips left. At 46mpg thats 3 gallons spread over 4 pips, or about .75 gallons per pip or 35 miles per pip. Thats not out of line. Technically to be consistent I should have spread that over 5 pips to account for the phantom top pip, but I'm assuming the dealer burned through most of the 11th pip during the first 36 miles. This would indicate that the dealer probably only put in 7-8 gallons, and/or burned through part of a gallon during test drives. Because the gas gauge has to try and figure out when the tank is really full, it can be fooled into acting like it was filled all the way up even if you stopped a little early.

I'd have to agree with Kcissem, I don't think you can really tell much from this tank since you don't really know what went in. My feeling is everything will be fine, as long as you have an appreciation for the Prius' eccentricities

Rob

 

I would have to disagree with that. I have, on 4 occasions, put in 12+ gallons(12.4, 12.4, 12.162, and 12.051) and one other time put in 11.3 gallons. Our own FireEngineer(Wayne Mitchell) has said that 13 or more gallons has been put into a North American Prius with a bladder.

 

That is larger than the gross capacity of the tank. With many cars you can add more gas than the tank holds by overfilling. There is some extra capacity in the filler tube, vent, and other assorted nooks and crannies. That doesn't mean it's a good idea, especially in a Prius, and it doesn't change the size of the tank.

Most Prius drivers will be lucky to ever get eleven gallons in a single fill.

Tom

 

You can't base any conclusions on the first tank of gas from the dealer.
You don't even know if it was really full.

You only have as much as fuel in the tank as YOU pumped in. So take it down to the lowfuel warning and fill it up . Hopefully you will pump at least 10 gallons in, and go from there.

Your range will always be 10X (or however many gallons your pumped)your average MPG for that tank. Displayed on your MFD, assuming you reset it.

Do that and you'll never have to contribute to another bladder thread again.!!

 

There is amazing variation in first pip mileage among Prius drivers. My small, statistically insignificant PriusChat poll shows that most drivers fall within the wide range of 100-200 miles for that first pip. The most likely mean is approximated in the range of 160 miles.

If you'd like to see or participate in the poll, follow the link to Max Miles on First Pip.

 

I wouldn't worry too much about the first tank. I'm on my second tank of a brand new 2009 Touring Pkg #3 (first post). I had 48.2 mpg on my first tank but only 456 miles on the odometer (translates to 9.5 gallons). This is reasonable, but it did seem a bit low to me. I'm wondering if the first filling tends to expand the bladder some so that subsequent fillings allow a bit more fuel.

In my first refueling, the tank took 10.505 gallons at the first auto-stop on the pump. I didn't bother to put more in at that point. My hope is that you'll have a similar experience.

I suspect that the bladder may become more elastic with time and may allow better estimates of fuel left. If you continue to want a better estimate, you might check into the ScanGauge. I ordered one from the PriusChat store shortly after taking delivery of the car and expect it should assist after the first two tanks of calibration.

 

That may be true under some circumstances, but certainly doesn't seem typical. Given that a number of people have damaged their cars by trying to overfill, it seems like a bad idea to encourage others to try doing so.

Rob

 

I have never encouraged anyone to do so. Although, if telling others that I have done this for 23K+ miles and usually do so with the car still in "ready" and haven't had any problems is encouraging them, then maybe you have something. I don't think so though.

 

Just use the gauge to know when you HAVE TO fill up. I mean you can fill up any time you want but it is a good practice to fill up if you get down to 2 bars on the gauge, fill ASAP when down to 1 bar and if that 1 bar is flashing you better get fuel NOW. Otherwise the gauge is just a count down to fill up time, ignore it as an economy gauge. The MFD gives an excelent comparison for you from tank to tank, just reset at each fill and try to beat your last "score" with each tank.

The bladder from all reports takes a little while to break in and will hold a little less fuel in the cold of winter. I have no first hand experience of the bladder as it only seems to be in North American Prius.

Enjoy your great new toy, learn a new way of driving and having fun while being safe and reducing your carbon footprint.

Here is something I think about when people complain about the bladder. My old car a 2 litre manual got 600km from a tank and cost $80 to fill, and that was using hypermile techniques. This car gets 900km from a full tank with fuel to spare and only costs about $62 to fill even though the price of petrol is higher now. So if I only got 650km from a tank I still fill up less often and pay less per fill. Of course if I filled every 600km I'd use 1/3 less between fill so I'd only pay about $40 a fill not $80 and I can fill in half the time.

I love my Prius!!