Navman Fuel Flow Meter Question

[Pat Anderson]

OK, this is just an observation, and I wonder if anybody has any explanation or thoughts.

We have the Navman 3100 gas fuel flow meter / computer. My main purpose was to improve economy (and secondarily to have a more reliable "gas gauge" and estimate of range remaining for the Alaska trip), so I usually have it set to show instantaneous mpg. I periodically flip over to gph to do sort of a manual check on the instantaneous economy reading, and it is usually pretty close. This is very good to show the relative effects of running at different rpms / mph. I also always check the gallons used and gallons remaining periodically, several times a day on the Alaska trip. You can definitely see your overall economy rise and fall as you adjust your speed.

One thing I have noticed though, is that there is a discrepancy between the the instantaneous gph and mpg readings, even maintained for a long time, and the overall average calculation you can make with the Navman at the end of the trip (i.e., Navman shows total miles traveled and total gallons used, so you can get an average mpg by simple division).

These may not be the precise numbers but are close and illustrate the issue. The instantaneous flow at 8 mph (don't bug me about mph and nmph!) might read 1.4 gph and the economy might show 5.7 mpg (it might vary from 5.x to 6.x). So I run for three hours, and this stays pretty constant (this is the trip from Gig Harbor to Shilshole). So I expect that the total gallons used will be somewhere around 4.5, maybe 5, definitely under 6. Instead the Navman reports the total gallons used as 8, and the calculation for the miles traveled shows the average mpg was 3.75. Now, 3.75 is a lot better than the 2.3 - 2.5 we usually calculated going faster, but not what we expected from slowing down this much. And this is NOT a Navman calibration issue - not related to the gallons required to fill the tank compared to the reported total gallons used - it is totally internal to the Navman (and the GPS connection). And from a calibration standpoint, as far as I can tell, we are conservatively calibrated - usage reported by Navman is set to be a little higher than actual usage on fill up, maybe 10%. So I generally do trust the gallons used / gallons remaining readings from the Navman (I will of course keep checking this).

Any thoughts on this discrepancy between instantaneous readings and calculated average results?

[thataway]

Pat,
This is the third boat I have the Navman 2100 on--so I have to do the manual calculations for MPG--(only shows gal/hr and total fuel used). My thoughts are that there is a little fluctuation in the flow meter--it is a turbine for those who don't have one--and that averaging interval of the GPS speed--combined this may give more variation than just one factor. In other words, both instruments have averaging intervals--and this can compound small individual varaiations. If I read your question correctly.

The bottom line is that the instantenous flow is important as for engine effeciency--and predicted fuel use (predicted Range)--but the really important number is amount used at the end of the day--between fill ups. If the amount used between fill ups (the calibration) is constant and accurate, then the turbine is reading correctly, and the variation is the GPS--and this may include currents, set, drift etc.

[Sawdust]

Pat,

Not a problem on statute miles - as long as you use knots in your car

Dusty