Steve
That would mean we should be posting the dash board numbers.


Maybe I should go back and fix it again.

 

kenny,
It WOULD, but then those w/o dashes would be skewed in the results...i'm just making a point as to why its different. I think the calcualated should be what is put in.
Cheers,
Steven

 

But this is a hybrid board
It would be nice to compare apples to apples.
Every hybrid has a dash MPG read out.

Pump numbers have a lot of variability because of pump inaccuracies, temperature during fill up, gas expands when hot, gas tank also expands at hot.
I read here that the federal government only requires the pumps to be accurate within 10%.
That means if the pump says 10 gallons, you may be getting anywhere from 9 to 11 gallons of gas.
I thnk we all know which side they error on.
That would explain why we all get worse MPG at the pump.
We are getting less gas than we think, and it is legal.

If every HCH owner put in dash reading it would be more fair and repeatable.
External variables are eliminated.

People who drive Corollas or other cars can enter pump data.

 

Next time I'm filling my 5 gal container for the tractor I'll pay more attention to how much the gas pump says at the 5 gal fill mark.

......Assuming my can is marked correctly!

 

Good idea.
But that will only tell us about one pump and one gas can, at one temperature.
More variables.

 

Hi Kenny:

___I have been filling 5 gallon cans and various cars for years and never had a pump display anything out of the ordinary other then the first time I placed almost 14 gallons into the Insight. Even that was explained with the 3 gallons heading into the vapor recovery HW. The displays are what are all over the place, not the tanks you fill them from. Over time whether a car has a bladder or not, cold or warm, filling to 1 click or right up to the filler neck, whatever … odometer reading/gallons filled over longer period (like that of the database) is more accurate then the FCD’s.

___Although my LMPG from the FCD is within .1 of my actual calculated LMPG, individual tank over tanks can sometimes be off as much as 2 mpg calculated vs. the FCD. Posting FCD numbers is the wrong thing to do given these inaccuracies. The Toyota Prius is a habitual over achiever in terms of the FCD w/ a usual + 2 to +3 over actual over an entire tank from what I have read. The Buick Le Sabre I rented last month was ~ 2 over actual. The HCH sounds a bit closer sometimes but it is + or – depending on whatever.

___Good Luck

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

 

I don't think you can generalize things to the Prius display being 2-3mpg over manual calculation. I have four tanks on record so far, two of them are over and two of them are under. As for lifetime, my number calculated from the consumption screen is .7mpg above gas volume calculation. Not 2-3mpg.

Given the way the the Prius calculates mpg number on the display, it can't possibly be all over the place. It's perfectly possible that there is a car dependent calibration error, but I think the numbers are stable enough for tank by tank comparison on the same car.

I don't know if you've actually filled a Prius before, but you can't see the gas level down the neck at all as there is a flap over the neck. If you start to force things, you can easily overfill and you won't know it until you pull the nuzzle out then the car will throw up all over you (don't ask how I know... :-). There are supposedly poeple who rock the car side by side to burp it (just like you burp a baby so he/she won't throw up as easily :-) Never tried that myself so I don't know how well it works.

Now if you keep things on the cautious side, you won't know how close you are really. These days, I squeeze in half a liter at a time until a little bit of gas bubbles up the flap and then go back down. But that is not precise as I have been able to do that twice in a row without the car throwing up. So there is a non-trivial slack in the "fill to the neck" method. As for first click, it's highly dependent on pump pressure. It won't work unless you go to the same station and same pump all the time. Most of us don't do that.

So manual calculation is good for lifetime because it doesn't have a calibration error, but the individual tank number is seriously in doubt, on the Prius at least. Thus the display number with correction factor method I propose eariler. But in the end, which ever method is fine I think. This stuff is not serious like the Olympics eh

Andy

 

My '96 Vette had a gauge that was about dead on and I trusted it more than the gas station pump readouts. The 2000 is always high, anywhere from 1 to 3.5mpg, so I don't trust it. Manual calculation is the only way to go one the 2000. I agree that fill up inconsistancies make single tank numbers meaningless but they are accurate in the long run.

BTW gas stations have their tanks undergroud and the temp of the gas is almost alway around 55deg. Do a search on ground coupled heat pumps to learn more.

 

Hi Accwai:

___I do not know if you have visited Priusonline or not but the stories of the FCD’s being off and/or varying gallons during tank fills are way to numerous to ignore. There are too many posts to dismiss so you are back to odometer miles/tank over tank gallons to get an exacting number. It isn’t the Olympics but you should strive for closer numbers so you know how much temperatures, traffic, techniques, or even setup is affecting mileage. We aren’t speaking of 1/2 the fuel economy unless you drive with your foot to the floor at 90 + one time vs. 50 + and being gentle but tenths of a gallon difference to see what helps and or hinders. If it were the Olympics, we would have our glass beakers out at each and every fill

___Good Luck

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

 

Actually, I did say both the consumption screen is off and the fill is inconsistent. So I'm not disagreeing. My last message was more about the nature of the consumption screen error and whether or not the fill can be made consistent, as you suggested.

For the former, the error on the display is such that it's more or less the same tank over tank, on the same car. That's what I meant by not all over the place. It might not be dead center, but it's at more or less the same place all the time. And apparantly it varies from car to car too. So you can't say all Prius are off by 2-3mpg.

For the latter, I've explained why you can't consistently fill a Prius either to the neck or to the first click. So that doesn't work either.

Now what we know is this: The number on the display is a good relative indicator for tank to tank comparison. The total miles/total gallons number is good if averaged over a lifetime but it can be quite misleading on individual tank. Therefore, my proposal was to computer a correction factor based on the ratio between lifetime mpg calculated from the display number vs. lifetime miles/lifetime gallon filled. Then you apply the correction to each tank mpg from the display. With this correction, you get accurate lifetime mpg as well as meaningful tank mpg. Best of both worlds right?

The only way this would not work is if the screen measurement doesn't remain stable from tank to tank. In the case, you have to explain to me where the error would come from. My understanding is that the Prius uses the number of times the fuel injectors are fired for fuel volume measurement. If that doesn't remain stable, I think the ratio of the combustion mixture delivered to the cylinder would be unstable as well. In that case, I think we have a much bigger problem then inaccurate mpg guage...

Andy