The Pump and the Nav - Comparison
 

Thought I woould start a new thread for comparing MPG calculations from the fillup and the navigation system. This way we don't have to read through all the convo's in the other thread.

10/24/08 34.06 Pump / 35.8 Nav = 5.10%
11/5/08 34.04 Pump / 35.5 Nav = 4.29%
11/20/08 32.37 Pump / 33.4 Nav = 3.17%

Re: The Pump and the Nav - Comparison
 

I'm glad I'm not the only one who likes to stir the pot every once in a while. Keeps things somewhat exciting around here.

By looking at your numbers, I'd have to say that your NAV readings are inaccurate by about 4% on the high side. Whether it be by the onboard computer or by a Scan gauge or any other device, the most accurate measure of what your vehicle gets is based on the following formula:

Total miles driven / total gallons of gas added to vehicle = average mpg

This formula eliminates the slight error caused by the differences in the level of fill for the tank. The more tanks you have in the data, the less that error becomes.

There is no way any device can get a more accurate measure of the average fuel efficiency than that. I'm sure someone will be along shortly to tell me I am full of it.....they always are.

Re: The Pump and the Nav - Comparison
 

You're going to find this interesting:

9/2/08 46.659 Pumped / 46.4 Nav = -.5%
9/18/08 45.86 Pumped / 47.0 Nav = 2.5%
10/4/08 45.173 Pumped / 47.8 Nav = 5.8%
10/25/08 46.04 Pumped / 47.8 Nav = 3.9%
11/14/08 48.06 Pumped / 50.0 Nav = 4.0%

These are all of my tanks 2-6 except the first because I reset the Nav after the dealership and the factory miles averages. My '09 FEHL MPG record is now logged in the data base at Clean MPG and anyone can view each tank average for more details. I keep my own log of the Nav and the SGII tank readings at fill-up.

My FEHL has burned 88.364 gallons since the factory start-up and I had 3,974.1 miles on the permanent OD at my 6th Tank. This gives my '09 FWD FEHL a lifetime average of 44.974mpg. I purposely averaged all factory start-up and testing, transport operations running and dealer handling and prep into my Lifetime Average.

GaryG

Re: The Pump and the Nav - Comparison
 

Very interesting. I'll bet our error rate will go up as soon as our FE goes up. Thinking I should send the wife down to you for lessons. lol

Re: The Pump and the Nav - Comparison
 

The ScanGauge does not take into account Fuel Trim.
The ScanGauge assumes you are using 100% gasoline.
Maybe the on-board MPG meter is the same?
I thought the on-board one would be 'smarter'.

People who get lower numbers at the pump, are probably using 10% ethanol.
10% ethanol lowers my MPG by about 3.5% to 4%.
On 10% ethanol my LTFT reads 3.5% most of the time.
The SG always reads 3.5% to 4% higher.

When I put E85 in my car ( yes, I do it sometimes, with no ill effects so far )
My MPG goes down 30%, and my LTFT goes to 28.5%.
SG shows normal MPG ( mid thirties ) when reality is mid-twenties.

Coincidence?
I think NOT!

My Nav MPG moves up and down a little with changes in ethanol, but not in the correct proportions,
So I don't know what the heck it is doing or how it gets it's calc.

P.S. Just for GaryG: I am POSITIVE the SG does not read the ethanol.... thus, at your MPG "Challenge" with Debbie's car, you got accurate results for what you WOULD have had on pure gas. The SG didn't know the car had E10. Next time, you could use E50 and get the same SG readings. Kinda good to know the fuel in the tank didn't skew participant's standings!

Re: The Pump and the Nav - Comparison
 

I thought that was Billyk.:zip:

Re: The Pump and the Nav - Comparison
 

Not quite sure how I'm stiring the pot with this thread. I opened it to fin out....
1. The error between the Nav & the Pump is not just on our Nav.
2. Average error % by compiling information from several vehicles.
3. What may be causing these errors. (GPSMan had a good theory about LTFT and Ethanol gas.)
4. Is there anything we can do to fix the Nav error or have this error fixed.
I probably have more reasons for starting this thread but I need to go stir the pot...err I mean brew some coffee.

Re: The Pump and the Nav - Comparison
 

"Baiting" ;) can be an important method to "catch" information and education. It is a credit to this site and it's members to have a free education clinic.

That said and respect to the previous post on scanguages:

Using mathematics relative to energy content per volume and mixing ratios, one comes up
with an energy content of 109,300 Btu per gallon for E10. This is approximately a 3.3

percent decrease in energy per gallon compared to 100 percent gasoline.”

If it is all about BTU's -energy content---then how does one lose more than four times (15%) this in MPG when running E10?:confused: It can't be bad driving can it? My personal experience does not match this type of difference. Heck, running E30 (5 gallons of E85 in a 15 gallon tank) does not drop my MPG 45%.

Re: The Pump and the Nav - Comparison
 

This topic has been discussed so many times over the years that I have been reading (and the couple years I have been posting) and it always creates the same debate about the scangauge and how to best measure fuel mileage, etc.

Sorry if you took my comment personally - it wasn't meant as a dig.:zip:

Re: The Pump and the Nav - Comparison
 

I know the effects of E10 compared to straight gas in my '05 and the SGII read the mileage lower as did my Nav Sys. My '05 fill-up gallons matched my SGII with E10 and straight gas perfectly. It still matches now that I've been running only E10, so there is no problem I can see with the SGII and ethanol. If my '05 was averaging 45mpg with straight gas, E10 would drop my average to 40mpg. If I worked real hard on my MPG, I could increase the 40mpg average to 42mpg.

During the MPG Challenge, Debbie had adjusted her SGII to read 7.5% lower than her Nav Sys. She had a real low tank fill and made the adjustment prior to the Challenge. The only effects of E10 was it lowered the mpg what we would have gotten on straight gas after the long term fuel trim was adjusted.

Our '09 FEH/MMH has a new 190 pin powertrain control module (PCM) and I read the dealership scan tools had to be calibrated for the new PCM. I have a feeling this is why I'm seeing much higher mpg readings on the SGII. No big deal because I can adjust the SGII to read correctly. I was working on finding the percentage the Nav Sys was off, but thanks to Mike and this thread, a few more of us can put our heads together.

Not sure the 11.1% adjustment to the SGII tank refill was enough though. Right now with 213 miles on this 7th tank, the SGII is reading 49.4mpg and the Nav Sys is reading 50.6mpg. If the nav sys is 4% high like we think, the SGII should be adjusted to read 48.5mpg.

GaryG