Display MPG number doesn't match actual mpg

<div class='quotetop'>QUOTE(SBaker @ Jan 14 2007, 04:46 PM) [snapback]375618[/snapback]</div>

Join the club. Remember that the gallons you fill is affected by the bag in the tank. I have observed that if you keep track of the MFD mpg and the calculated mpg they converge. It is a confusing system but in the end the gallons you use and the miles you go is equal to the mpg. There may be a song in this... oops the Beatles all ready did this!

 

The best way to determine MPG is over the life of the car. I've recorded every fillup and average about 44 over 11000+ miles.

You can't really worry about the per-tankful figures. The MPG can vary among tanks due to different fill amounts, weather, terrain, etc.

 

<div class='quotetop'>QUOTE(Rick Auricchio @ Jan 14 2007, 05:52 PM) [snapback]375649[/snapback]</div>

Exactly. The "actual" mpg isn't actual. Even if it's off, the MFD mileage is a lot more (at least I hope so) consistent.

Dave M>

 

The MFD MPG is based on the time your injectors are open divided by the rotation pulses from your wheels. The pulse-width of the injectors is constantly adjusted for minimal pollution and optimal fuel efficiency. Wheel pulses are for both forward and rearward motion and presume a stock wheel.

Gasoline, at least in the US, is not dispensed at constant temperature/volume. You get more gasoline energy in your gallon when the gas is cold and less when it's hot because gasoline expands and contracts with temperature but a gallon is the same volume an any temperature.

So, your MFD MPG is far more accurate in the short run, and converges with calculated MPG, within a couple of percent, in the long run.

 

Does the fact that the MFD rounds up to the highest mile (e.g. ODO says 23.2 MFD says 23) have anything to do with this as well?

My first inclination is that this contirbutes to the discrepancy depending on what tenth of a mile you're at. Or is it just too insignificant?

 

Being that "engineering accuracy" is oft quoted as +or- 5%, I think the reported readings from the MFD are pretty good.

There are a lot of variables:
1. Fuel bladder in the fuel tank will not allow you to consistanty "fill" the tank to the same amount.
2. The fuel you buy does change over the seasons in energy content.
3. The fuel you buy expands and contracts with temperature - a lot, actually.
4. Because the Prius has a "small" fuel tank (I'm comparing it to the anti-Prius -see below-) even small differences in fillups can give you perceptively large differences in mileage.
5. The air used in the engine varies in density with temperature and although the engine tries to compensate for this, I suspect it's not perfect. This can result in differences in combustion efficiency.

All in all, the MFD readings are amazingly accurate. Just try to buy a mileage meter that accurate for less than $1000!

 

<div class='quotetop'>QUOTE(hdrygas @ Jan 14 2007, 07:37 PM) [snapback]375644[/snapback]</div>

lol

<div class='quotetop'>QUOTE(Rick Auricchio @ Jan 14 2007, 07:52 PM) [snapback]375649[/snapback]</div>

Yeah, I usually compare my per-tankful figures over something like the last 10 fill ups, and if I see a consistent drop it's usually because I need to throw some air in my tired.

 

<div class='quotetop'>QUOTE(Bill Merchant @ Jan 14 2007, 09:00 PM) [snapback]375724[/snapback]</div>

Uhhhhh . . . . what? :blink:

I don't understand this statement. You say gas expands and contracts with temp, but then say that it's the same volume at any temp. EVERYTHING expands when warm and contracts when cold. HOWEVER, liquids generally have a very LOW thermal expansion. The Prius system cannot possibly reflect the higher mileage that cold gas would provide. It would need the mileage registered out to several decimal places before you would see the difference.

The fact that most folks, for many reasons, get lower mileage in cold weather would seem to support my point.

 

<div class='quotetop'>QUOTE(garbageman @ Jan 19 2007, 01:23 PM) [snapback]377939[/snapback]</div>

And as well, most gasoline is stored in tanks well below ground level, where the temperature is nearly constant year round. In the time it goes through the pump into the gas tank the temperature doesn't change that much.

Dave M.

 

After nearly three years and 32712 miles, my display average is 53.6 and the calculated is 52.59. That's close enough for me. As many here do as well, I have recorded every fill up.

 

ditto here-
over 18,000 miles with resets only afew n the beginning and now the MFD is approaching my figures. I think my bladder has finally been trained......lol :lol:
I was planning on a reset at 20,000 and starting again.
what ever you do- just enjoy!!!!!!!!

 

Well, I checked and gasoline expands about 0.0006% per deg F around room temp (the rate changes at other temps). A petroleum engineer tells me that most of the expansion we see is caused by air trapped in the tank - it expands at 0.002% per deg F.

Further, in the US your fuel is dispensed through meters that are not temp. compensated. Here in Canada our fuel meters are temp. compensated. There are stickers on them saying they are tested at a specific temp.

One reason Canadian fuel dispensing meters are temp compensated is our temps. vary a lot - I've seen -50C here and +40C. That's a pretty wide range! In fact one spring day a number of years ago I saw a 40 deg C swing in one day (from +20 C to -20 C)!
The other reason is our govt. mandated temp. compensated meters.

 

Re: Display MPG number doesn't match actual mpg

I reckon the MFD mileage reads about 10% fast ( only because I think the speed is 10% fast too) so if you think you're getting 44 mpg in reality it could be about 40.

I've tried using a sat nav but that tends to take straight line distances bewteen a number of points so is always much less than the MFD

The only way of doing it is to measure a calibrated circuit with a calibrator of some sort.

If you fit larger diameter tyres you'll think you are getting worse mpg because the sytem will measure less miles between A and B - whereas the higher gearing should improve the mpg - and in practice it probably does

 

Re: Display MPG number doesn't match actual mpg

It is a trivial matter of programming to exaggerate the *speedometer* reading. The displayed MPG uses the *odometer*, which by law must not be deliberately inaccurate. If you're using the correct tires the odometer and the MPGs are quite accurate.

 

Re: Display MPG number doesn't match actual mpg

I'm not sure how to confirm
The miles travelled per tank given on the mfd is always a few miles different to the difference in odometer readings I note at each fill up

Does that give a clue

 

Re: Display MPG number doesn't match actual mpg

Well that is odd. My mfd miles is the same as the trip mileage rounded down an integer (I reset the trip at each fill-up). I don't normally look at the total odo reading, but it agrees with the sum of the trips over the car's lifetime to within a few miles.

 

Re: Display MPG number doesn't match actual mpg

With 3.5 years of data my bias on the MFD is 1.5 mpg higher than reality. That's on an actual 53.9 mpg lifetime average (2.8%).

I record every tank and track it (Paul's Prius Home) even accounting for outdoor temperature. Check out the graphs and charts in the link above.

There are variables with every fill - pump accuracy, bladder issues, etc. The MFD gives a good tank to tank indication (especially after you measure and know it's bias). You need to log all fillups to know your true lifetime number.

 

Re: Display MPG number doesn't match actual mpg

Since I got my 2007 Prius one year ago, I have kept track of fuel added at each refueling and calculated the mpg since the last refueling by dividing the miles driven by the volume of fuel added. For the many reasons given by others, calculated results of any single fueling rarely agree exactly with the mpg shown on the MFD. Since the car's reported distance (rather than independently measured miles driven) is used in both calculations, that potential difference is removed.
At each fueling, I also calculate the MFD-predicted gallons used by dividing the miles driven since the last fueling by the average mpg shown on the MFD for that period.
Adding all the fuel added (as indicated by the fuel pump delivering the fuel) over the 9,100 miles and dividing that into the total miles driven (as indicated by the odometer) yields an overall average of 48.91 mpg. Adding all the MFD-predicted fuel added (calculated as in the preceding paragraph) and dividing that into the same number of miles yields an overall average of 50.13 mpg.
So, in my case, the mpg indicated by the MFD, on average, is about 2.5% optimistic, compared to dividing actual fuel consumed (as indicated by service station pumps) into miles driven (as indicated by the car's odometer). I don't consider that to be a reason for concern -- it's well within the accuracy that should be expected.

 

Re: Display MPG number doesn't match actual mpg

There must be a thread here that says how the mfd knows how much fuel is being used - does it use injector on time?

Even more of a mystery is what measures the amount of fuel in the tank with the bladder in the way.