Computer estimated gas mileage off.

[Laserbait]

Every time I fill up, I reset the trip computer, the average fuel economy, as well as the fuel used.

The average fuel economy reading is always about 0.5 mpg better than what I actually get. Also, the fuel used is less than what I actually put in the tank.

Is there anything that can be done to make the average fuel economy reading accurate?

 

[Flag Gibby]

What you describe sounds about right for the way my computer calculations have been ever since I went to a more aggressive all terrain tire on the vehicle. I am sure that if I switched back to the OEM style tire that the calculations would be spot on. I have just adapted to how much the calculations are always off and automatically adjust for it if I am looking at them (MPG and fuel consumed). The other thing that I have noticed that impacts the calculations is the amount of ethanol in the fuel that I put in the tank.
So to answer your question, there is really nothing you can do to make the calculation more accurate.

 

[edwx]

Also maybe the mpg's on the computer is calculated by an algorithm, and isn't an accurate mpg from fillup to fillup.

 

[Laserbait]

It just seems so weird that the "fuel used" in the computer is less than what is actually used to fill the tank.

 

[gadabout]

There is no guarantee that the tank will be filled up to exactly the same level at each fillup. There is also no guarantee that the vehicle is sitting on a perfectly level surface each time you fillup. All these variables, and others, will affect the numbers. Also, how do you know "what I actually get" is correct and not subject to measurement errors?

 

[Laserbait]

I wasn't looking for guarantees, I'm just wondering why the computer thinks it's using less fuel than it actually is.

I know what I actually get for MPG, because I track it. Figuring out gas mileage isn't tough. Take the number of miles traveled, divide by the number of gallons used at your fillup, and the result is your actual gas mileage.

Over my last 60 fillups, the computer has always shown less fuel used than I put in the tank, by about 0.5 gal. The reading is off when going to the same gas station, same pump (did that 5 times in a row to rule that out), different pump, different gas station. This leads to 3 possibilities that I can think of:

1: The gas tank is slowly getting larger, and hiding that "extra" half gallon.


2: There is a very slow fuel leak that I can't see or smell.

--I doubt this, because my long trips, where I run through a tank in a day, the measurement is still off by about the same amount.

3: The computer is assuming something about the fuel system that is not correct.



So now my question is, how does the computer determine the amount of fuel that it's using? To me, if I were doing the programming, I would use the rail pressure, injector flow rate, and the injector pulse duration and rate. To me, I would think that, this method should provide the most accurate measurement.

Now, in my case of "3" above, if lets say, the computer is assuming that the engine has 35Lb/Hr flow rate fuel injectors installed, but for some reason my engine had 38Lb/Hr injectors installed. This, I think, would cause what I am seeing.

 

[gadabout]

I'm only suggesting here that if you're making the calculations yourself based only on a single fillup, then the result you get will only be approximate. For one thing, the amount of gas that you probably had in the tank is probably not exactly the same as what you had before your prior fillup. Also the level of gas after the fillups is not going to be exactly the same.

Of course if you are tracking this over say 10 or more fillups and use the total number of gallons and the total mileage traveled, then you would have an accurate result.

 

[swshawaii]

Your Fuel Economy Gauge Is Fibbing

http://www.edmunds.com/fuel-economy/your-fuel-economy-gauge-is-fibbing.html

 

[Laserbait]

What you describe sounds about right for the way my computer calculations have been ever since I went to a more aggressive all terrain tire on the vehicle. I am sure that if I switched back to the OEM style tire that the calculations would be spot on. I have just adapted to how much the calculations are always off and automatically adjust for it if I am looking at them (MPG and fuel consumed). The other thing that I have noticed that impacts the calculations is the amount of ethanol in the fuel that I put in the tank.
So to answer your question, there is really nothing you can do to make the calculation more accurate.

Flag, sorry I missed your post earlier. I too went to new tires in 265/60R18, but the old bridgestones in 265-65R18 exhibited the same issues. Interesting to note about the ethanol. Too bad I don't have a way of getting non-oxi fuel here, legally.

I was hoping there was something that I could do - but I guess it is just an annoyance.


Thanks!

 

[Laserbait]

http://www.edmunds.com/fuel-economy/your-fuel-economy-gauge-is-fibbing.html

Interesting! I'm surprised, in this litigation happy country, that some ambulance chaser hasn't sued over this.