Hi Accwai:

___Here is a question for you. Does your FCD mpg for 4 tanks = what your total miles/total gallons purchased over 4 tanks? The second is accurate. The first is off for many reasons. IIRC, all of the FCD’s use injector pulses for their calculations. Do the calculations have density compensation based on temperature? How about fouling of the injectors themselves? Any pressure compensation as the fuel pump ages? Miles traveled/gallons purchased doesn’t have these kind of issues other then loss of ~ 7/32” radius as the tires wear toward 2/32’s tread depth. If the fueling station pumps you use are within that 1% number previously discussed, the manual calculation is the only true number you can use because it is in fact a real number and not calculated from a number of variables that are themselves varying as temperature or age change them.

___Good Luck

___Wayne R. Gerdes
___Hunt Club Farms Landscaping Ltd.
___Waynegerdes@earthlink.net

 

Like I said earlier, my mpg from the display over 4 tanks is .7mpg over the total miles/total gallons.

As for injector drift, I don't have any solid statistics on that so there might well be long term effects, I don't know. (Do you have any stats?) But what I do know is that with the bladder on a Prius, there is no such thing as a mark that you can try to fill to. My estimate is that the slack is at least 2 liters. 2 liter out of 45 is 4.5%. Personally, that's a much more immediate problem than the injector drifting. Yes, the error will even out over time and you have accurate lifetime number, but the inidividual tank numbers are indeed "all over the place".

I think using this type of tank data to benchmark technique and condition changes is misleading. For example, the calculated mpg for my first tank is actually higher than my second tank. Which is clearly not the case, as I was much more consumption conscious on the second tank. I think the reason is the pump I used for the first fillup was really weak. It probably cut off about 4 liters early and I didn't know how to force it at the time. And of course the second tank took the hit for that.

Anyway, I don't know what more I can say on this topic. It's an imperfect situation all around. I just want to make the best of it...

Andy

 

Great discussion here, a credit to the knowledge of the posters!
It is about which is a more reliable data, the dash or the pump calcualtion.

Steve.
I quoted you when I thought pumps could be 10% off.
Do you know what your source was?
Thanks

One thing seems clear, the dash *almost* always reads higher.
The gas stations have a financial incentive to adjust their pumps to give us as little gas as legally possible.
Call me a skeptic or cynical but I think that assuming gas stations are ethical, and pumps are accurate, is naive.
99.999% of consumers don't verify pump accuracy, and they know it.
Then again, car makers have an incentive for getting the dash to read a little high.
They look better, and customers are happier.

Very good points brought up about car variables like tire diameter wear and diameter of engine parts.
However I think we are looking for the largest variable.
Considering the consistent direction of the error, and the financial incentive I think pump accuracy deserves our main attention.

We need a source for pump accuracy.
Also information on enforcement, or lack thereof.
Who does it, state, federal, county?
How often?
Are they slacking off from budget cuts?
What do they find?
Is there corruption?
Follow the money! I say.

I'm sure some are thinking, "Man! why take it so seriously?"
Answer: "I don't know."
It's all for fun, actually.

 

Hi Accwai:

___Let me put it another way. My Insight’s lmpg is dead on. My segment can vary +/- 6 mpg depending on how deep I can fill her vapor recovery HW. If it’s a full fill (~ 13.9 – 14 gallons) the next tank with a short 11.0 gallon fill causes extreme anomalies of course which I have to average so the “Real Hybrid Mileage Database” isn’t skewed nor is my own knowledge of what the Insight is worth form the FCD or not. You know what a 10.8 gallon fill over 1,400 - 1,500 mile tank looks like? Believe me, the mpg over a tank achievement record would look like it was created by Bozo the clown if I placed them in there without averaging the two

___Back to my point, If your Prius is within .7 mpg to the + side at each tank fill over the last 4, you are not having a bladder induced error as it is dead on (over the last 4 tanks anyway). With that, if your FCD is .7 mpg over actual miles traveled/gallons filled, it is .7 mpg over reality period. Your data is to close imho not to believe actual calculated vs. the FCD. Real miles traveled over real gallons filled is the most accurate way to measure because FCD’s are not that close. I still don’t see why you would use the FCD knowing full well it’s already consistently > .7 mpg above actual today? When you keep your spreadsheet over 10’s of tanks, you will more then likely see the FCD not meeting reality more then what you have posted so far. The threads in regards to this in the other forums about the Prius II in particular are to overwhelming to ignore as stated above. In other words, the FCD will induce errors over the short and longer terms that cannot be corrected accurately. The Miles traveled/gallons purchased will not need any correction because they are actuals.

___Kenny, in regards to pump accuracy, there is one item overhanging each and every station owners head. If the scales inspector shows up and you are pumping that 1% off, you will be paying a nice little fine and having someone come to re-rig the pumps at your expense. Do you think BP is going to deal with a law suit from your truly for an over/underage of .3 gallons/20 gallon fill? Not a chance and I know what my various tanks hold and what they don’t. Anyone showing up with a 5 gallon can will know instantly that there is a problem and if they perceive the problem and it’s reproducible, in comes the local news team with an investigation. $2.00 +/gallon gas brings this kind of fraud to the forefront might quick if you know what I mean There is to much attention to this for a station owner to play games with and I suspect most don’t have the expertise to jerry rig the later pumps anyway given the all digital nature.

___Sorry all if I am coming across less then understandable. I had a nice 16 and 8 hour shift in the last 36 hours with a hell of a drive on both sides and in between.

___Good Luck

___Wayne R. Gerdes
___Hunt Club Farms Landscaping Ltd.
___Waynegerdes@earthlink.net

 

Sigh... Could you show me where I said to go with the screen number and ignore the lifetime manual calculation please.

While there is a .7mpg difference between display and manual over 4 tanks, the difference is as high as 4 mpg on individual tank. The latter fluctuation is far too high to provide insight into tank over tank changes. And we have a perfectly good explanation as to why is so high too. To me, claiming it doesn't need correction and not doing anything about it is unacceptable.

I never said to use the display number straight. Instead, the display number should be corrected using a calibration factor. The factor is the ratio between the lifetime numbers from manual vs display. This factor is recalcuated and reapplied to display tank data on every new tank, so things are always up to date. This way, the .7mpg difference is fixed, and the 4mpg fluctuation is fixed as well. It might not be perfect, but I believe this is closer to reality than either straight manual calculation or straight display number.

I've repeated this quite a few times already. If this doesn't sink in, I don't have anything more to say...

Andy

 

___I never said ignore the lifetime but you are trying to ignore the individual tanks and go with a much less precise FCD X a correction factor which in itself can be off as much as 7% by your own reckoning. 4 mpg off vs. your actual average of 57 mpg as an example.

___I asked you how your last 4 tanks actuals were in comparison to your FCD and you said they were all off by .7 mpg. In further detail you said they are all over the place by 4 mpg.

___This appears similar to mine and everyone else with an FCD from my understanding. I am off at worst 6% of actual in the winter/spring temps and I do know my tank holds ~ 11.0 gallons without a vapor recovery fill? Placing correction factors over many tanks does not rid the actual mpg shown of a single tank outlier. A correction factor on a variable that isn’t consistent leaves you with all kinds of inaccuracies and they are not always correcting to the right side of actual. An example. Let us say you have a + 4 mpg on your next tank which would apply a negative correction factor to the next tank(s) following even though the last tank fill after the first + 4 may be negative. If it is negative, then what do you have? This will skew this last tank even more! A 7% FCD display discrepancy is almost as good as the Insight’s 6% max that I have seen in mid-winter and again, I can tell within a few tenths of a gallon on a non-vapor recovery fill if actual makes sense before I actually do the calculation in my head or by the spreadsheet when I get home.

___I do need sleep but I also know FCD’s are not that accurate over a single tank. It is one thing if it always + 2 to + 3 but it isn’t always + 2 nor is it always even a + on a tank over tank? Well not on my Insight anyway so maybe with the Prius it is different? You will have to tell me if you have received a + .7, + .7, + .7, + .7, and a + 4 vs. actual. With that, it sounds like you need a CF of ( – 1.36 ). With the negative CF, what happens if your next tank is – 2 per the FCD vs. actual? In other words, the FCD shows 55 vs. an actual of 57 but the RMDB will add the negative CF to give an actual 53.64? Eventually the CF will turn out to equal a true number to match the LMPG but it still doesn’t give a single tank outlier any more chance of being correct.

___Another example … Let us imagine it is tank # 100 and you have a CF of ( - 1.3 mpg). If you ever get a lesser FCD vs. actual (it is bound to happen) even after 100 tanks, then that individual tank is as bogus as no CF applied at all?

___I am also not a statistician so don’t take my thoughts on this as proper for the discussion. I just see the Insight receiving great summer time mileage with the FCD being very close to actual. In the Winter, the FCD has a bit more variance from actual but not enough to concern me knowing after another tank, they will average out correctly or darn close anyway.

___Good Luck

___Wayne R. Gerdes
___Hunt Club Farms Landscaping Ltd.
___Waynegerdes@earthlink.net

 

Sorry I misread. I did say .7mpg was lifetime display vs. lifetime calculation and the four tanks by themselves are all over the place. By the way, the 4mpg is an absolute value. I didn't imply they're all positive. In fact, I did mention earlier that two are positive and two are negative.

<!--QuoteBegin-xcel@Aug 22nd 2004 @ 10:44 PM
[...] I can tell within a few tenths of a gallon on a non-vapor recovery fill if actual makes sense before I actually do the calculation in my head or by the spreadsheet when I get home.
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You see that's the difference between us. There is no such thing as consistent fill on a Prius. I would be really lucky if I can get close to half a gallon consistency tank over tank. When you see a difference between the display and the tank, you automatically assume it's the display number that's the problem. My assumption is that the tank can't be filled to a high degree of accuracy and therefore the display is a more stable base for tank to tank comparison. So I apply one correction factor, based on lifetime differerce, to all tanks. The factor is recalculated and all tanks recomputed on every fill. So the corrected display tank consumption and tank distance yeilds a lifetime mpg that is exactly the same as total volume/total distance calculation.

Since we cannot agree on what causes the difference between the display and the tank, I don't see we'll ever come to a closure on this one. I see no point continuing like this.

In any case, I've been using display numbers in the database so far because the display vs tank fill difference is quite small. I will start applying the correction factor on my next fillup. So by that time, lifetime number on JHX will be exactly the same as manual calcuation.

Andy

 

This is going nowhere.
How about two columns?
One for dash
The other for calculated at the pump.


Then all the information is there and each reader can draw their own conclusions.