Error in average MPG display

I was really pleased to see what high average mpg I was getting on the display in my brand new OBW (2006 2.5i, manual trans). 28.5 mpg and up on my regular commute of about 22 miles highway and 3 subarban/city.

Then I filled the tank. The display, which I had reset when I first filled the tank, said 26.1. The number of miles divided by the output of the gas pump came to 24.35.

This is a 6.7% difference. In the wrong direction.

It's only one data point, and I will of course, check after each fill up for a while, but it leaves me wondering what other people are getting when they compare the mpg display to real consumption, and what people here, and Subaru, think are reasonable tolerances.

Charlie in de Bronx

Reply to
shopsis
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" snipped-for-privacy@gmail.com" writes: (snip)

It might partly be a distance-measurement error: the speedometer and trip counter in my WRX seem to be a bit off. (I should check the odometer with my GPS to make sure I'm not being penalised for too much mileage!)

-- Mark

Reply to
Mark T.B. Carroll

IIRC I8 has signs that precisely measure 3 miles. This is old school compared to a GPS. Every odometer I've had read a couple tenths over 3 miles. I'd just love to be there when someone returns a lease with over miles and demands a correction for an optomistic odometer. Let the fireworks begin!

Reply to
speednxs

Of course, the error from tank to tank will vary depending on the pump and other things that change the amount the tank will hold. The true error can be calculated by comparing over a handful of tanks.

My analysis is like this: the computer knows many things with great accuracy. It knows how far you have gone, normally with the same sensor output that drives your odometer so any error nulls out. It knows precisely how often it fires the injectors. What it knows with less accuracy is how much fuel is injected with each firing. It knows how much it asked for, but it makes assumptions about the viscosity of the fuel (probably a good assumption) and the fuel pressure.

I think most of the error is in minor inaccuracies in the fuel pressure regulator.

Mike

Reply to
Michael Pardee

Don't forget gas station fuel pump accuracy.

Some fail inspections. Fuel delivery speed sometimes affects accuracy.

Reply to
y_p_w

I looked at this a few months ago and decided the pumps were *supposed* to be accurate enough that they wouldn't account for the several percent discrepancies most of the time. Of course, there will be exceptions....

Mike

Reply to
Michael Pardee

I have noticed the display on my car ('06 Tribeca) reads 0.5 to 1.0 MPG higher than what I calculate from miles driven and gallons purchased. The error seems higher in colder weather. Could it be the computer doesn't register gas used for some period of time after each start?

Reply to
Ian

These things are toys. They are not expected to be precise. They give a general impression, that's all. Useful for comparison purposes - ie seeing if you can improve your mileage by adjusting your driving style

- but don't ever take them seriously as a measure of consumption.

Reply to
David Betts

The only one I have is on our 2002 Prius. It's pretty accurate, always 3% to

5% optimistic. Considering all the inputs to the calculation are digital and the assumptions are typically good to better than 5% it's really no surprise. Overall it is about as accurate as the speedometer.

The gas guage is notoriously inaccurate in the Prius (owners often call it the "guess guage") and the MPG display, including the miles since we filled the tank, is the best way of determining how much gas is actually in the tank.

Mike

Reply to
Michael Pardee

I have a Tribeca and it is very consistent in a 1 MPG error. I always enter all gas purchases in a spreadsheet and it gives very accurate measurement. When the Tribeca shows 19.7 MPG for a tank, for instance, the spreadsheet calculation will show 18.7, give or take one tenth.

Don Dunlap

Reply to
Don Dunlap

Reply to
Bryan J. Lee

Hi,

The way one such system was described to me is that it "measures" the fuel flow rate thru the lines and compares that to the speed of the vehicle in making its calculations. If the car's sitting, there's a "divide by zero" kind of situation, no? If so, then I think you're on target with your thoughts!

Rick

Reply to
Rick Courtright

There is no division by zero. The calculation is miles traveled divided by fuel used. The numerator is zero while you are idling but the denominator is not.

When you are going down a hill, THEN the numerator is much greater than the denominator and the instantaneous display will go to its maximum value and then stop displaying, at least on my Passat.

The time at idle should be correctly included in the average MPG calculation.

-- Vic Roberts Replace xxx with vdr in e-mail address.

Reply to
Victor Roberts

I noticed this morning, on two occasions while I was idling at a red light with the car in neutral, that the "average mpg value" decreased, suggesting that gas consumption while idling is included in the average.

I'll do a more focused experiment, when I have a chance, by pulling over and idling for several minutes shortly after the car has had a long enough trip for an average trip mpg to register on the display.

Charlie in de Bronx

Reply to
shopsis

Obviously you are correct, but someone else here had that same observation and came to the conclusion that the idle time is not included.

I have seen this many times in our Passat - as is to be expected.

-- Vic Roberts Replace xxx with vdr in e-mail address.

Reply to
Victor Roberts

Stopped idling time affects correct, real world MPG. While stopped and idling, the car is getting zero MPG. On the same token, during a long descent, the engine is using idle fuel at highway speeds, creating a period of high MPG.

If only moving time is counted in the display, it's a gimmicky toy.

Reply to
Bonehenge

The idle consumption pretty much has to be included. MPG readings can't be averaged, so the total miles has to be divided by total gallons.

Our Prius has to "dummy up" the readings when sitting - much of the time the immediate consumption rate is actually zero, and even zero divided by zero is undefined.... The interval readings are correct, though. At speeds over

11 mph, if the engine hasn't started, the display just shows 100 mpg rather than trying to divide by zero.

Mike

Reply to
Michael Pardee

Mike:

With the Prius does the actual mpg (miles per tankful)/(gallons/tankful) agree with the car's calculation?

Charlie

Reply to
shopsis

It's pretty close, running 3% to 5% optimistic. I normally think of the error as 2 mpg; like 47 instead of 49. I don't check it much any more, but did check it out the first few tanks. I've occasionally thought I could calibrate it by tweaking the fuel pressure regulator about 1 psi lower, but the risks seem higher than the value of calibrating the display... especially since it's mainly my wife's car.

Mike

Reply to
Michael Pardee

11 mph, if the engine hasn't started, the display just shows 100

Hi,

Just out of curiosity, does the engine start at a given speed (or pretty close), or does it somehow figure acceleration rate, cruising speed or other factors into its "decision" when to start up?

Rick

Reply to
Rick Courtright

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