Gas Slips or on board computer?

<div class='quotetop'>QUOTE(FishHawk @ Jul 14 2006, 12:46 PM) [snapback]286355[/snapback]</div>

I calculate my MPG per tank and life-to-date according to the gas slips. I also note what the MFD read for each tank as a point of comparison. But in the long run, total miles driven divided by fuel gallons replaced should be accurate.

P.S. Nice screen name!

 

The manual calculation is what goes in my logbook. The computer calculation is interesting, but over the long haul, only the manual calculation is to be trusted.

However, I will say that among cars we've owned with a fuel consumption computer, the Prius is closer to reality more often than any other car...

 

<div class='quotetop'>QUOTE(jrct9454 @ Jul 14 2006, 02:42 PM) [snapback]286420[/snapback]</div>

After 26,000 mi in my 2005 Prius, I have the same results.

 

I track the fuel receipts at the pump.

If you want better bragging rights, go with the mfd. On my car, the mfd is about 2 to 4mpg higher than reality.

If you think someone on here is lieing about the mpg they are getting, it is possible they just tell you their mfd number. They may believe it to be equal to their actual gas filled and thus, not be lieing on purpose.

My car does not get as many mpg as my mfd claims, so I don't follow it down that ugly 'web of lies' road.

52.21 mpg over 17296 miles over 148 days.

http://d.spicher.home.comcast.net/prius/LifetimeStats.html

 

A while ago, I asked the question on which was more accurate, and the consensus of those measuring it over 20K miles or more was only about a 1% difference between the two. As of late, I have been taking the MFD rather than the calculated. Mainly because of the large differences in temperature between fillups that I often see in Lubbock. With only filling up once every 3 weeks or so, the temperature could vary 30 degrees between the fill-ups (and affecting the bladder). These temp changes generally lead to a MFD that was much lower than calculated (7-10mpg lower).

Now that summer is fully here and the temps are more consistent, it probably really doesn't matter. Either way, I have a pretty good idea of what the mpg is for the car after 36K+ miles, so I don't think using one or the other is going to affect the lifetime that much.

 

I've read several posts regarding how MPG is measured on the Prius. Most of of those posts mention that the computer caluculates MPG using the measured injected fuel. If this is true, you can't get a more accurate MPG calculation than what's on the MFD.

 

How do you know the gas station fuel pumps are accurate?

my dentist, who drives an Insight, once put almost 12 gallons of gas in his car (it has a 10.5 gallon tank). It was quite an intresting story, actually. The county auditor could not do anything about it because he did not have a pay-at-pump reciept. he had a reprinted reciept from the attendant. (He then went on to calculate that this unethical practice would make the gas station around $10000 additional profit over a weekend)

 

Calculated from receipts with handwritten notes. I just use it to monitor driving techniques mostly and to figure the running average miles per gallon.

 

Can't see how to do it manually when I can't tell if the tank bladder is actually all the way full...

 

after 14319 miles, by hand: 52.96, by MFD: 53.95 close enough for me.

if i were to say what i got for a tank, i'd say what the MFD showed, but lifetime i'd use the by hand figure.

<div class='quotetop'>QUOTE(jmann @ Jul 14 2006, 11:28 PM) [snapback]286579[/snapback]</div>

see here
another thing they get people on is the 9/10 of a cent.

 

<div class='quotetop'>QUOTE(Rufaro @ Jul 15 2006, 07:54 AM) [snapback]286658[/snapback]</div>

I do it manually with an EXCEL spreadsheet, but also track it over 2, 3, 4 and 5 fillups and cumulatively, and compare it to eh MFD readings for each tank and cumulative weighted-average MFD. It's so easy to do with EXCEL.

I did this with my old car, as well, for a few years - even produced some graphs from it. I live in NJ where we don't pump our own gas, so even with a 'traditional' gas tank, I could never guarantee how 'full' it was. The single-tank MPG varied quite a bit, but the 2 tank average was reasonably smooth.

 

<div class='quotetop'>QUOTE(Jeannie @ Jul 15 2006, 07:01 AM) [snapback]286703[/snapback]</div>

Tee hee--this gives me a very twisted image of a 21st century version of the 50s live television commercials, where the actors on the show would have "spontaneous" chats about the sponsor's product...or Betty wotsername demonstrating a refrigerator and stuff like that.

Of course, this could be because I've been up all night again... :huh:

 

<div class='quotetop'>QUOTE(Rufaro @ Jul 15 2006, 04:54 AM) [snapback]286658[/snapback]</div>

There is no way to get an accurate figure for any given tankfull, but if you keep careful track of all the gas you put in your car over a long period of time, and if the gas pump is accurate, and if your odometer is accurate (or you know the exact percentage it's off) then you can calculate your average mpg over that longer time period.

For myself, I am far too disorganized to actually keep track of my gas. I tried at first, but after forgetting to write down the numbers several times, I gave up.

I am concerned for the environment, and so I drive a Prius. But it makes no difference whatsoever to the environment if I know or not what my exact mpg is!. So in that sense, I don't really care what my mpg is.

And here's the kicker: Some people, wanting better mpg, and knowing that short trips are mpg killers, will drive more miles in order to get their bragging rights up, and in so doing burn more gas and cancel out any benefits they'd have otherwise done to the environment by driving an efficient car. Thus the numbers game is often counter-productive.

 

Gas slips!
HOWEVER- I never take one tank as being 100% accurate. This is a perfect example:

Last tank:311.1 miles, 4.779 gal = 65.097 MPG. Display now reads 48.5 my lifetime according to miles/gas = 46.89 best previous tank was 52.9

I also have suspicion that attendant did not top off tank because I can usually drive about 150 miles before 1st pip disappears, today it went after 100 miles, so in effect actual mileage on that fill up may be more like 54 MPG....

 

<div class='quotetop'>QUOTE(Jeannie @ Jul 15 2006, 07:01 AM) [snapback]286703[/snapback]</div>

Hey Jeannie--would you mind pm-ing or emailing me a copy of your spreadsheet...I know there are links to other people's, but yours sounds a bit less complicated and easier for me to understand...and <strike>steal</strike> adapt for my use. Now, having added up the gas I've bought (and estimated what was in the not-quite-full tnk when I bought the car), I am coming up with about 42mpg over about 1500 miles, which seems to more or less agree with what I keep seeing onboard...It WILL get better, right?

 

The Prius measures the amount of gas injected into cylinder 1 and multiplies it by four. There's no way that gas slips can be more accurate. Folks who do all these manual calculations are simply carrying on with a behavior they developed back when there was no better alternative.

 

I use the consumption screen.

Because of the bladder and not being able to use the same pump at the same station every time, I don't think the gas slips are any more accurate or inaccurate than the consumption screen.

 

I track my mileage on a spreadsheet, and I "reverse engineer" my MFD mileage to get a lifetime figure. The MFD lifetime is pretty consistently *around* 2% higher than what I calculate using my gas receipts. I try to fill up at the same pump each time, because there are a couple of pumps there that don't fill as full.

 

<div class='quotetop'>QUOTE(jamarimutt @ Jul 16 2006, 03:48 PM) [snapback]287268[/snapback]</div>

That's great if you don't ever accidentally reset the touchscreen (or did it before you knew better, like some of us... ). Or IS there a way to retrieve lifetime mpg after the dreaded reset?