actual VS computer

[barbaram]

I gave up a while ago on the MPG as calculated by the car's computer. It NEVER agrees with my actual number= mileage divided by the gallons...... I know there is much debate on account of the bladder.

Does anyone know which is really more accurate????????????

[john1701a]

<div class='quotetop'>QUOTE(barbaram @ Jun 1 2006, 09:11 PM) [snapback]264406[/snapback]</div>

Does anyone know which is really more accurate????????????
[/b]

It all depends on what type of data you are actually trying to measure.

If you just want a quick "what has recently happened", the computer is practically dead-on... if you first subtract the pre-determined rounding factor. For my HSD Prius, that's 1.3 MPG.

If you want an overall "lifetime average", the on-paper calculation based on a collection of at-the-pump measurements is the only truly accurate method.

[miwaku]

<div class='quotetop'>QUOTE(john1701a @ Jun 1 2006, 10:28 PM) [snapback]264413[/snapback]</div>

If you want an overall "lifetime average", the on-paper calculation based on a collection of at-the-pump measurements is the only truly accurate method.
[/b]

I disagree, John. I'm up to seven tanks so far, and the numbers are starting to indicate things.

MPG and Avg are based on hand calculations from the odometer & gallons indicated at the pump.
Tgals is Total gallons pumped so far.
MFD is reported straight from the screen.
CGals is based on the number of gallons the MFD seems to think the car used (Miles/MFD).
TCGals is total gallons the MFD thinks the car's used so far.
CAvg is is (Odo/TCGals). Sorry, Tochatihu; you're right, that's the best way.

Odo__Miles__Gallons_Tgals___MPG__Avg__MFD__Cgals__ TCgals__CAvg
_321__321___7.005__7.005__45.82_45.82_46.2__6.948_ __6.948__46.20
_789__468__10.468_17.473__44.71_45.16_45.9_10.196_ _17.144__46.02
1057__268___6.297_23.770__42.56_44.47_43.5__6.161_ _23.305__45.35
1428__371___7.598_31.368__48.83_45.52_45.1__8.226_ _31.531__45.29
1699__271___6.599_37.967__41.07_44.75_45.9__5.904_ _37.435__45.38
2125__426___7.502_45.469__56.78_46.74_45.9__9.281_ _46.716__45.49
2480__355___9.674_55.143__36.70_44.97_46.1__7.701_ _54.417__45.57

Of note:
1. Compare Gallons to Cgals and you'll see the MFD usually thinks I use less gas than I do, but looking at tanks 4 & 6, it seems obvious that I wasn't able to completely fill the tank, especially tank six, since tank seven got REALLY bad numbers. Tank seven, BTW, is where the pump stopped after 3.5 gals, giving me only half a tank, and I had to drive to another pump and add another 8.5 gals. Compare Tgals (55.143) to TCgals (54.417) and you'll see it all seems to average out in the end.
2. My lifetime average is just under 45mpg; the MFD says it's 45.57. Not too much of a difference!

I've just switched to the MFD's numbers rather than my own. I think I trust it more.

[john1701a]

<div class='quotetop'>QUOTE(miwaku @ Jul 23 2006, 09:16 AM) [snapback]290920[/snapback]</div>

CGals is based on the number of gallons the MFD seems to think the car used (Miles/MFD).
TCGals is total gallons the MFD thinks the car's used so far.
CAvg is is (Odo/TCGals). Sorry, Tochatihu; you're right, that's the best way.
[/b]

Huh? What the computer thinks!

You cannot rely on that estimate. Just look at the 117,000 miles of data I've collected to find out why.

The measurement of how much was actually pumped into the tank is the only appropriate number.

[Betelgeuse]

<div class='quotetop'>QUOTE(john1701a @ Jul 23 2006, 11:14 AM) [snapback]290942[/snapback]</div>

Huh? What the computer thinks!

You cannot rely on that estimate. Just look at the 117,000 miles of data I've collected to find out why.

The measurement of how much was actually pumped into the tank is the only appropriate number.
[/b]

I wonder if you can explain why the computer can be off by so much? I definately believe your results, since you've certainly averaged over enough tanks to get rid of the tank to tank fillup variations. However, it would seem to me that the computer should know exactly how much gas was burned, since it needs to calculate how much gas needs to go to the injectors to keep the ICE running smoothly.

Any ideas?

[miwaku]

<div class='quotetop'>QUOTE(john1701a @ Jul 23 2006, 11:14 AM) [snapback]290942[/snapback]</div>

Huh? What the computer thinks!

You cannot rely on that estimate. Just look at the 117,000 miles of data I've collected to find out why.

The measurement of how much was actually pumped into the tank is the only appropriate number.
[/b]

I thought so too, until I looked at my whole 2,500 miles of data and saw that, while it was obvious to anyone just reading the few lines of data that I wasn't filling up the tank exactly the same every time, even after such a short time, the numbers tend to even out. But you're right; let's look at your "lifetime" data...

According to the data on your website (http://john1701a.com/prius/spreadshe...t_Lifetime.xls), your lifetime average through 59,827 miles is 45.48, and the MFD says it should be 47.49.

Then I checked your other lifetime sheet (http://john1701a.com/prius/spreadshe...t_Lifetime.xls). Lifetime for the next 57,117 miles is 48.69, compared to the MFD's estimate of 49.98.

Personally, I'll stick with "what the computer thinks." I think it's entirely possible that it's a little more accurate than my pumping skills. Thanks for all your help & advice, John, but isn't it possible that a machine that comes SO CLOSE to your own estimates might possibly be at least somewhat reliable?

[john1701a]

<div class='quotetop'>QUOTE(Betelgeuse @ Jul 23 2006, 10:27 AM) [snapback]290944[/snapback]</div>

it would seem to me that the computer should know exactly how much gas was burned[/b]

What is the precise size of a squirt of fuel, under each type of operation condition?

To what digit are the calculations carried before rounding or truncation occurs?

Seemingly simple tasks get really complicated really fast. So the level of accuracy we enjoy we difficult enough to achieve. It's darn close. But expecting it to be dead-on isn't realistic.

[miwaku]

I agree, which is why I'm letting the computer do the calculating for me.