Actual vs indicated mileage: Prius not delivering ?

Can you explain it again in english? lol

It sounds like you are trying to put more fuel than it will hold by assuming you should be able to put 11.9 gallons inside the tank.

Have you searched all the threads on this subject yet? There are a TON of them with good information on data collected over long periods of time (many tanks).

 

Do NOT overfill the gas tank! Gas can get into the charcoal filter and mess up the pollution control devices (fume recovery), and that can cost a lot of money to fix! I go to the second click just to make sure the first wasn't premature, some people don't even advise doing that. If you use the same station, same pump, you don't need to top off, it should be pretty consistent.

If you put more gas in than the time before, your computed gas mileage will be lower than actuality. It's only when you fill the tank to the same level as the previous tank that you will have an accurate measurement. Consistency is more important for accurate numbers than trying to "stuff" in as much as can fit (or more). Your "indicated" (on MFD?) levels are probably closer to reality.

Plus, those mileages are not bad mileages. My first tank was 40.5 mpg, then I went into the mid-40's, lifetime average of 49 mpg now.
Your mileage will depend on your commute (longer=better mileage, but more total gas consumed, I can tell your commute is at least 5 miles each way), weather (SF not too extreme, should be pretty good there, but the hills might be a problem), driving style/traffic patterns (want to slow down naturally for all stops as much as possible, and time the greens), tire pressure (inflate several pounds over the minimum PSI posted on the door frame). Summer numbers should improve, as long you don't need too much A/C.

 

You should check your tire pressure while the tires are cold, I think the recommended (door panel) 34 front 32 rear but you should do no less then 38 front 36 rear.

 

There is to much variation in the Prius gas tank to do the comparison you are trying with such a small sampling. As of my last fill-up I have 136,986.70 miles on my 05 package 9. I have filled up the car 375 times and used 3,367.679 gallons of fuel.

I calculate the MPG in 4 seperate ways:
Total miles/total gallons of fuel = 40.67688755
MPG Shown on MFD = 41.42586667 (this is taken from the consumption screen each time I fill-up)
MPG calculated at each fill-up with the fuel put into the car during that fill-up = 40.62019163
MPG calculated at each fill-up with the fuel put into the car during the previous fill-up = 40.65848494

The last 2 methods are just checks to show potential differences due to the wide variable in the amount of fuel the Prius takes on each fill-up. From largest fill-up to least fill-up (both taken at close to the same level of empty on the tank) I have had a 5 gallon swing in amount of fuel the car took.

There is still about a 1 mpg difference between what the MFD shows and the calculated amount, and given the checks I would say that the cars calculated MPG is actually off.

 

To reiterate what nerfer said: Do not overfill!

The bladder can and will split, causing a very expensive and not-covered-under-warranty disaster.

Do a search using the term overfill in these forums to learn more.

 

Don't top off, and don't bother with manual mileage calculations, unless you are willing to do them over a large number of tanks. The volumetric uncertainty introduced by the fuel bladder makes manual calculations a useless exercise. By totaling a large number of tanks, you spread out this error and reduce it to a manageable level. How many tanks depends on how good of numbers you need. I use the MFD calculation, as it is fairly accurate and requires no effort on my part.

Tom

 

Ze_shark... For whatever's it's worth I'm copying a post I made last week for your perusal. Take heed of the warnings in this thread... Best, Wes Shaw
Earlier post follows:

—Just visited this 'overfill' thread for the first time and will add, for those 'keeping track,' my experience with overfill.... New 08 Prius in November. Located in Eastern Coastal Maine. On January 4th, low barometer, outside ambient air temp of 25º±, on my seventh fill at an RH Foster (chain) station and pump I've used with no problem for 20+ years, the following occurred:

Tank was down to the blinking bar so I, in retrospect, should have expected 9 gal.±..., inserted pump nozzle, set to middle automatic position (auto-fill is legal in Maine...), started pump, picked up sponge washer, did headlights, refreshed squeegee, started on tail lights and the pump clicked off. I observed only 7 1/2 gal. on pump so I started adding manually and very slowly in that I was new to the car and had a 'vague' idea that I'd read the bladder tank was a slow fill and, from the manual, was aware of not attempting to "top off" as I'd done for a lifetime in my other vehicles... My mind (I'm age 67) wandered off, I guess, and suddenly I began to smell gas. I stopped pumping and observed a considerable puddle under the rear of the car —it did not come bubbling and burping out of the fill pipe. I was shocked to observe 11.3 gallons on the pump! (Again, for a lifetime, I've been used to using the autofill to first shut-off, and then manually adding gas through successive auto shut-offs until I hear the gas in the fill pipe. ——Perhaps this has been, in retrospect, an 'unsafe' practice but I believe it is quite 'common' throughout America and I would respectfully suggest that Toyota address the issue...) Letting everything sit and evaporate, I blocked off the pump area and went in to advise the manager that I believed the pump nozzle auto shut-off was defective. She advised that the pumps had all been recently checked and certified and that no one had reported a problem with that pump before. —That I should have stopped filling after the first auto shut-off....

Quite angry, I eventually left the station and drove an extra hundred miles to draw down the excess fuel in the hope that I'd not done damage to the filters, etc... In my cooling off period I concluded that she was correct. I should have stopped after the first auto shut-off (even though it appeared the tank was one to two gallons shy...).

On subsequent fills I then adopted the policy of filling manually and on the slowest flow. On the first auto shut-off I stop and check the pump and then the dash gauge. Usually, I will see all the bars lit and about what I expected for gallons on the pump. However, I have noticed a few times that the first auto shut-off occurs and there is still one unlit bar on the gauge... In that case I proceed to slowly add a bit more with a careful eye on the pump gallonage and any sound at the fill pipe. I've not overfilled again (8 tanks) using this method.

Lastly, now with a Scan Gauge II, I've enjoyed noting the predicted gallons needed on the SG II's read-out and have been very happy to note that the pump auto shut-off occurs within ± one tenth of a gallon of the Scan Gauge's prediction.


Conclusion: To avoid overfill and/or damage to the 08 tanks (at least) and systems: fill Manually, fill Slowly, Anticipate (by car gauge, paper calcs, or Scan Gauge II) the gallons needed, and Observe the pump gallonage at you near the top... "MSAO" to add a new acronym to the Prius glossary of terms??? ( : >)

It would appear that this tank overfill thing is a very real problem with the Prius but I'm so extremely happy with the car that I've learned to not mind the 'extra' time it takes to fill the old-fashioned way.... It will be interesting to see what any investigation(s) produce....

My time to re-explore threads is sometimes limited so please do not hesitate to e-mail me at: [email protected] if you would like or need any corroborative confirmation of my logged overfill noted above.

Best, Dr. Wesley A. Shaw

 

Not to get too far off topic, but after reviewing the other 'gas overfill' posts and having experienced the 'burping' of an overfill about 5 times in my 375 fill-ups, I have the following opinions. (if others have experienced differently I'd like to know)

Overfilling of the fuel system should absolutely NOT cause the types of damage being discussed, specifically, ruptured bladder or contaminated vapor canisters! (I'm not saying that this hasn't occurred, just that it shouldn't. And I haven't had that experience)

Overfilling fuel is relatively common and should be designed with that in mind. To design a system that:

1) allows overfilled gasoline to enter the airspace between the bladder and the tank is negligent at best and incompetent or worse! Fuel in such a space could quickly vaporize becoming an explosive risk. (given that an explosion has never been reported I would say that was a very low possibility, but still no fuel should be allowed in that space no matter how much fuel is pumped into the car.

2) no matter how much fuel you put into the bladder it should NEVER rupture (barring extreme age) the tank surrounding the bladder should be designed to prevent such an occurrence. If not that is a flagrant design flaw.

3) the overflow relief tube of the fuel filler nozzle should be designed in a way that does not allow fuel to fill the vapor canister. Again, with the commonality of fuel overfilling a design that allows for such contamination is negligent.

I would like to know if anyone has fought Toyota on these issues. I would be surprised if they stuck to the hard line that you overfilled the 'gas' tank causing damage to the system that wasn't covered under warranty.

 

This would only occur if the bladder ruptures.

It's a bladder; a balloon on steroids, if you will. Over-inflate it and it will rupture; no getting around that.

Ideally, there shouldn't be any reason for fuel to go through there, but pumping excessive amounts of fuel into the tank will result in the fuel going somewhere it isn't normally supposed to go.

Let's assess: the owners manual clearly states that one should not "top off" the gas, that there is a bladder in the tank that can and will be damaged by overfilling.

Warranties, as a general rule, are voided when the user adopts methods and practices that run counter to those specified in the manuals and instructions provided with the product.

Thus and therefor, by ignoring the warnings and strictures in the accompanying instruction booklet, the user who overfills the vehicle expressly voids the warranty.

To imply that the manufacturer is liable for the misdeeds of its customers is, in my opinion, asinine and wrongheaded. Is overfilling as "relatively common" (I'd like to see the statistics on that) as, say, running into a wild animal in the road? Does that mean the car maker should be liable for damages because they don't include a properly working animal avoidance system?

Should Dell be held liable for damage to my laptop because I spilled milk all over the keyboard? After all, spilling liquids on keyboards is a relatively common occurrence; using your logic, then, dictates that Dell should have factored liquid spillage into the design of the keyboard and thus I should sue for damages.

 

Dngrsone:

Generally what you are saying appears logical except for maybe something you missed in my tirade above:

1) your are correct you cannot infinitely fill the bladder, HOWEVER why in the name of Satan's sphincter would you put the bladder in a container (the actual tank surrounding it) that would allow the bladder to rupture? Especially of the reason you designed the bladder was to avoid gaseous gas produced by sloshing fuel.

2) It makes some sense that one should not overfill the gas tank, however it is way too common an occurrence to not be taken into consideration when designing the fuel system. And given the vagaries of the Prius fuel system (which produce the "burping" effect caused by the bladder, overfilling is, I believe more likely in the Prius than any other vehicle!)

 

I agree that overfilling to a dangerous extent of the Prius is more easily accomplished than with any vehicle that does not have a bladder.

Keep in mind that overfilling is a largely American habit, and the Japanese engineers who built this vehicle might not be familiar with our tendency to treat our fuel systems in that way.

Take into account, also, that just because we have always done it that way in no way makes it mandatory for any auto manufacturer to become complicit in our abuses.

New variable-capacity fuel tank vehicle owners will just have to break themselves of a bad habit or risk the consequences.

 

Also doesn't every single gas pump have this warning printed on it.
WARNING: DO NOT OVERFILL OR TOP OFF
sometimes its even printed on the nozzle as well, so it seems to me like its common problem in most cars.

 

Here are a couple of pictures to aid the discussion. Particularly note the second illustration.

 

It doesn't hold a lot of fuel, the sooner you realise that the better. Stop trying to squeeze in more and more fuel. Just fill it till it says "that's enough thankyou" then hang up the hose. It isn't that hard.

Don't fill up in ready mode, yeah I know the engine is off but turn it off anyway. Push the hose right in OK.

Rocket science it isn't.

Fill up when there are 2 bars on the fuel guage, I don't know of a special prize for squeezing a couple extra miles from a tank.

 

Patsparks, do you have the bladder in the Aussie version?

 

I have never heard that the bladder will rupture. I think that what will actually happen(instead of bladder rupture) is the gas will expand, go into the charcoal canister, and eventually make it into the space between the steel tank and bladder according to the information in past threads.

 

No but you only need to read the existing threads on PC about it.
Even with a bit of variability in tank capacity it isn't that bad if people fill up at 2 bars.