Not sure if this is a FEH/MMH issue or a more general issue...so I'll start this here:

I just fueled up my third tank yesterday. My NAV avg. MPG was saying 25.0mpg, I was happy, but when I did a calculation from actual odometer and total gal of gas bought from my receipt, I was getting 282mile/12.4gal = 22.7mpg. That's way off the 25.0mpg on my screen!

Is the NAV's est. normally this bad, or am I a victim of a cheating pump?

Could it possibly be that I only needed 11.3gal but the pump charged me for 12.4gal? It was hot, 98F, but not THAT hot.

This is the first time using this Shell gas station -- cheapest I've seen so far in So.Cal (in Hungtington Beach).

I filled up twice before at different gas stations, and I dont remember (being surprised) it being this way off...I would have had the same reaction if it were! Esp. when my first two tanks' NAV readings gave me lower mpg than I expected and I wasnt surprised that the actual calcuation is around the same for that first two tanks. Unfortunately I did not write down or keep any records for that first two tanks....but it couldnt have been that much off.

Anyone with experience with the FEH/MMH NAV's estimates, or cheating pumps?

 

I don't have the factory NAV in my 05 FEH(I have a MUCH better aftermarket GPS/NAV), but I have noticed the average mpg gauge in mine is slightly optimistic.....

~John

 

It is very unlikely to be a cheating pump - especially in Hungtington Beach - there is recent sticker from the Dept of Weights and Measures on the pump certifying its accuracy. What is more likely is that you didnt fill the previous tanks fully - Ive noticed that the Escape tank seems to generate more 'false fills 'for gas pumps and sometimes the pump clucks off when its not full. You cant reliably predict from thepump clicking unless you go to the same pump twice.

 

Hi,

You are assuming that 12.4 gallons is getting you back to exactly the same spot (tank fill-wise) as you started meaning that you consumed 12.4 gallons. It is quite possible that you are a bit fuller than when you started if, for example, the car was slightlhy tilted, or the auto-stop at the pump was less sensitive than the last time etc... There is no way to know exactly how much gas you consumed by how much you put in. Maybe after summing your values over 10 or more tanks you might statistically reduce that variable from the measure to be insignificant.

For example, if when you fill, you might put one gallon more or less than the previous fill, then for 1 tank, you consumed 12 gallons +/- 1 gallon for X milles driven, so 1/12=8.3% uncertainty, in your case 22.1mpg +/- 1.9mpg; not too far off the computer's number. If you sum it over 10 tanks, you will end up with something like 120 gallons +/- 1 gallon (1/120=0.83% uncertainty) with 10X milles driven. Your MPG calculation will be X+/- 0.19mpg, a more precise measure.

I hope this helps,

Regards,

rcomeau

 

Hi OCC

The FEH and MMH all have the same shape gas tank that has a large air pocket above the fuel neck connection at the tank. Depending on the slope the FEH is sitting at the pump, this air pocket can be filled and add up to a little over 17 gallons total to your tank. Sometimes you can only get a total of 14 gallons in the tank but the manual says it is a 15 gallon tank. There are shut off valves to the vapor recovery system which also prevent over filling of the tank. If the FEH were overturned, these valves would also prevent liquid fuel from entering the vapor system near the engine.

Generally, you have 1.3 gallons left at 0 Mile To Empty. If you drive till your at 0 MTE and fill the tank, your lucky to add 13.7 gallon to fill the tank. However, it is possible to add as much as 15.7 to 16.0 gallons at that time also. You can see from this that it is impossible to measure a single tank for your MPG. I'm sure this is what happen with your mileage this time.

If you reset the Nav or message center mileage average after fillup, it is pretty close to giving you your tank average. The problem is that the amount of data stored in the FEH system will start to drop off after about 10hrs into the tank fill. Some of us use the aftermarket Scangauge for determining a more accurate reading. The only thing I found is the OBD 11 connection give data in a "Metric" scale. Each time the data is converted by the SG, the "Standard" conversion reading is rounded off. This may cause the mileage in the trip OD in the FEH to be a little off on the SG reading. On a 600 mile tank, I have seen as much as 10 miles less on the SG trip.

GaryG

 

Now that everyone told you how you filled the tank incorrectly , I will tell you that yes, the nav system is off. Your results seem further off than is normal, but 1 mpg is fairly common. Very likely it is some of the nav system, and some of the tank fill.

 

My cars must be the exception because both of mine are within 0.5-0.8 mpg when comparing the dash gauge (non-nav) to the miles divided by tank method. I reset my gauge at every fill-up and only fill my tank to the second click to avoid overfilling. Most gas stations (around here anyway) have the filling area pretty much level and I almost always use the same station so I get a pretty consistent fill every time I think.

Eight-tenths of one mile per gallon works out to be about 2% error - to me that is hardly significant.

I wouldn't panic over one low tank or get too excited over one really great tank because it is probably just due to variations in the filling. The next tank will average it right out.

 

I have found that long drives tend to be more accurate than short ones. So if your average driving is for longer distances, it will be more accurate. It seems to go farthest off when driving in ev mode.

But I agree that it's not panic worthy. With a typical 1 mpg off, I generally don't care. The slightly annoying thing is that I've never seen it tell me 1 mpg low, always 1 mpg high.

 

My results seems WAY off!...and on the high side to boot...

I guess it just may happen to be all the worst case circumstances in my case for that particular fillup, but almost 10% off; 2.3 mpg off! That's a lot I think.

I do reset after every fillup.
I think I will keep track of the station/pump# and Nav readings for one station, then go back there and get a few consecutive tanks for that station/pump#. If I find a positive correlation...

 

I just went back and checked all the receipts for the last few tanks in each car and here is the comparison between what the gauge read and what the numbers calculate to.

2005 32.2g/31.3c 35.8g/33.9c 31.8g/31.9c 32.6g/32.3c 33.0g/32.9c 37.7g/37.6c 38.5g/37.7c

2006 33.0g/32.2c 36.0g/36.3c 36.8g/36.8c 35.0g/35.4c 36.7g/36.3c 35.7g/35.6c 34.3g/34.6c

When I first had these cars I used to try to get as much gas in it as possible. Since I started stopping at the second automatic shutoff, the difference between the gauge and the calculated number is much smaller.

I wish I had tracked this from the beginning - it would make a good data field in my ever expanding spreadsheet.