Fuel Gauge

Why would topping off or not have any affect at all on MPG?

 

Humm..... I know there is alot of discussion on the subject.... here is one thing I found... but directly relating to MPG, but emmisions..

From johns page Prius http://john1701a.com .. see "User-Guide" at the bottom.
Once in userguide look about 1/3 way down under "in green" Nurturing.. it talks about filling the tank.
In that same section you can scroll down to "Emission Bladder" also

Don't! Once the pump automatically stops, don't trying squeezing any more gas into the tank. The bladder will stretch, but it may spit gas back out if you force too much.

Also, pumping too much gas into the tank will result in gas filling up the vapor-capture canister. That defeats part of the emissions system. So if you want your Prius to remove the cleanest possible, avoid ever doing this.

John also believe alot to the fluctuations is due to outside ambient temperature affecting the stretchablity of the bladder?
http://john1701a.com/prius/prius-data.htm

 

This was a service campaign from about one year ago. If a customer complains about an inaccurate fuel gauge or backsplash, the dealer will replace the filler neck and load new software. It takes a couple hours to do, and, judging from reports here it seemed to be more trouble than it's worth.

I won't have it done. When my gauge gets to one square I refuel, slowly, and am happy to put in 7 or 8 gallons. No backsplash, no running out of gas. Life is good.

 

I understand that the amount of gasoline you can fit into the tank can vary because of the bladder. But it goes against common sense to think that the amount of gas in the tank has anything to do with how many miles per gallon your car will travel. MPG stands for miles per gallon, not miles per tank. The only reason I can think of that the amount of gas you have in your tank would affect your gas milage at all would be the weight of the gas.

What am I missing?

 

It does not affect the actual MPG, but it does affect the manually calculated MPG.

Let's say you drove 500 miles since the last fill up. The pump stops after 8 gallons. That calculates to 62.5 MPG. If you were to top off and squeeze in another 2 gallons, your MPG calculation will drop to 50. Of course, this will only apply to the individual tank.

 

True.. but if you "consistently" filled the same way each time wouldn't you be ok?.... bouncing back and forth is what would mess you up?

 

Maybe nothing.... I went back and did a search for "topping off" and didn't really find much indicating a drop in mpg... unless it eluded me... my memory was funky!
Most of the conversation was about vapor return and spillage into the environment etc.
So unless someone else remembers... nevermind!

 

That makes sense. Thanks for the clarification.

 

First off, when a fuel pump is functioning properly, I don't understand why anyone thinks they are smarter than a vehicle manufacture and they keep trying to fill up after the pump automatically stops. Do you really think they would invent and impliment such a device if it didn't serve a purpose? Are you a scientist? What the bloody hell? Stop f'in topping off and bi&*hing about the consequences afterwards!

Second, I don't know how people are claiming to get 200 miles for the first blip on their fuel gauge. You must have one jacked up gauge. Last night my gauge went down the first notch and I was at 83 miles getting 53 MPG. This seems like where it is supposed to change at, no?

 

In the debate about when to fill up, some have said to just use the MFD for your calculations. I think that might be risky, and I would give at least a 10% margin of error for that MFD. I found that it indicated 57 mpg, but when I filled up the actual milage was closer to 52 mpg.

But I too was interested in the range after the last pip blinks, because I've let it get so low too. I figured that there was between 25 and 35 miles before it would run out. I've actually gone about 25 miles on the blinking pip, but when I refilled it still took less than 10 gallons. (It was my first over 500 mile tank!)

So theoretically it wouldn't have run out for another over 85 miles if the tank holds 11.7 gallons, but of course the problem is that Toyota gave people a tank that can't use the entire amount, and maybe can't really be filled to the maximum amount. And it seems to vary too depending on the circumstances. To me that is the real reason there are so many Prius owners running out of gas.

Possibly a bit off topic, but it seems that years ago they were selling a small half gallon or so container of something that you could use instead of gasoline, so it was much safer transporting it all the time in your car, and then you wouldn't be stranded. But I never see anything like that anymore.

 

Walking down the rabbit trail with you!.....
Humm.. something I have heard no one ever mention?
I"m not sure how much gas weighs per gallon.. but water is 8.3lbs per gallon. Adding an additional 10 gallon tank would add approx 80 lbs to the rig..... like the trucks to... you would have a switch to change over when the other one runs dry..... this would allow over 1000 miles of driving between fillups... really nice if you find that really low priced station!..
I don't know if there would be issues with vapor etc... all else fails you could tap into the existing system and use without a switch?

 

Warning... I am an engineer!

The interesting thing this brings up is how to do the calculations. This is a little hard to explain in text, but I'll try

I now have two data pages on my spreadsheet. One calculates the "normal" way where I fill in wach row as I put gas in. (Page "Data" on my spreadsheet.) The scond one calculates how far I went on this tank based on how much gas I put in last time. ("Shifted Data"). I fill in everything as normal, except I put today's odometer reading on the previous tank. The gas I put in today has no miles on it yet. Calculating this way moves the variability away from how much gas I put in, and moves it to how much I had left when I filled. 90% of the time, I fill at the beginning of the trip on my way home from work, when the tank is at one pip.

On the "tank miles per allon filled" graph, you can see that the shifted data trendline (the linear one) has a formula of 49MPG(Filled) - 28 Miles. To me this means that I average 49MPG, except I take a 28 mile hit to the tank because of warm-up. Percentage wise, this works out weel with the % of my commute spent warming up (about 8 minutes out of 60). Notice that this trendline is close to the green line, which is just a linear plot of my current MPG average (unshifted, but shifted and unshifted MPG are almost the same).I've also drawn in the 50MPG "target" line.

On the non-shifted trendline, it indicates that no matter how much gas I put in, I get 270 miles free, plus 17 miles for every gallon filled. This is obviously a less accurate (logical) way of figuring out how far I can go per tank.

For both linear trendlines, the R squared value is barely better than .6, meaning there is alot of fluctuation in my datapoints, but I think it's OK.

The polynomial trendline on the shifted data is less clear as far as the formula, but it gives a better R squared value, meaning it fits the data better. Theoretically, I would always want to be on this poly plot, closest to the 50MPG "target" line to get the best MPG. This point is at approx 8.75 Gallons, where I should get 420 miles per tank. This should get me up to 48MPG in theory.

Obviously, I need more datapoints.

 

I updated the "Tank Miles per Gallons Filled" Chart to include lines for 35 through 60 MPG in 5 MPG increments, and tried to make things a little easier to see. It looks a bit confusing at first sight, but I made the datapoints smaller to be easier on the eyes. The important thing is how the pink line lines up with the teal line, showing how the "shifted" method calculates (seemingly) better per tank MPG.

Most of the graphs are based on the "Unshifted" data, however, since it's a lot of work to update them.

Are there any more correlations I should plot?

 

Wow!...I'm not too proud to admit you lost me awhile back! :lol: :lol: :lol:
But I do admire your charts.... I know they make sense to you and I appreciate your attempt at explaining them... I suppose If I studied them long enough I might soak up a little bit.....
Very impressive.....
I assume you work for Toyota.. if you don't, I hope your not working for the enemy!!! :huh:

 

I actually work for a company called Flow. We can cut almost any material using ultra high pressure water, with added abrasive (sand). Good explanation here. Our equipment is used to cut composites (like carbon fiber) on every single production composite plane manufactured in the world (or so marketing tells everyone) The only thing I've ever heard of that can't be cut is tempered glass, and noone can really cut that.

As far as the charts, what part do you not understand? I would be more than happy to explain more. By plotting diferent variables vs other variable, we might find some combination of data that shows a good way to improve MPG. (Like my theory of always filling up with 8.75 gallons)

 

I've heard about the cutting technology you speak of.. its quite facinating!

I will need to study your charts more before I give you some decent feedback.... I need to give them an honest chance... they are quite overwhelming!
Thanks...

 

I appreciate revisiting fuel gauge pips. My first pip goes at ca. 125 miles. I drive the same route each day (60 mi round trip) and the central pips "average" about 50 miles + or –. I can easily judge if I am going to earn 600 miles on a tank. Because of repeated discussion here about running out of fuel, I always fill up when the last pip starts blinking.