Garmin Fuel Tracking Issue

[it4llc]

After almost running out of fuel when we first got the boat by using the Volvo fuel gauge, I've relied on the Garmin fuel gauge on our 2020 C28. When we fill up, it's usually within a few gallons of what I expect it to be. I usually set the tank to full after pumping, knowing it's probably a gallon or two less, but I never go below 30-40 gallons in the tank so that was never a concern. At least until the last fill-up. My Garmin fuel gauge said I had 35 gallons left, but when filling up, instead of adding 65 gallons of diesel, it ended up being closer to 85. I don't know how the Garmin gauge would be this far off though as I thought it was generally fairly accurate, and not off by 20 gallons on a 100 gallon tank. Any thoughts on what would cause this?

 

[Bosn Mate]

I have an R29 Sedan and a few seasons ago I experienced a similar situation - the Garmin indicated that the engine burned a certain amount of gallons and I filled up close to that amount. Later, when I was ensuring the tank was full for winter storage, I added at least 25 gallons more than what the Garmin display indicated should be needed.

I believe the reason is simple. Over a period of time, I was not filling up to capacity and then resetting the Garmin to a full tank. It may have been only 1-2 gallons but over time it adds up.

The solution for me was simple. I use the Garmin burn amount as a starting point. Then I continue to fill until I get a healthy burp out of the fuel tank vent (rag in position to catch).

Mike

 

[SKing]

The tank is long and flat with the fill at the back of the tank, so if the boat is sitting stern low or high it will make a difference. As stated above always fill to the burp and then reset.

The Garmin fule indicator has always been accurate for me. 3ys and 600hrs.

 

[it4llc]

When I fill, I usually do it 'visually' meaning although I'll use what the Garmin gauge is telling me as a starting point, I fill until I can see the fuel sloshing on the (inside) top of the tank. That being said, I'm now second guessing whether or not I did that when we did a long weekend on the bay, which was the last time I filled up. We only filled up a few times but maybe it added up to enough. Going forward I will make sure I do the visual fill 100% of the time, at least when going below half a tank.

 

[tlkenyon]

2015 R25sc. I had kinda the same "how much is really in there" issue on our Great Loop trip. When we got home, I emptied the tank using a fuel transfer pump. Tilted the boat (on the trailer) bow up to make sure all the fuel was gone. Used a bore-scope to confirm empty. Leveled the trailer and then added 5 gallons at a time. Stuck the tank after each 5-gallon increment to directly measure the fuel depth. Continued to process until 75 gallons (capacity is 80). Also got the tank dimensions and did the math. Generated a graph.

r25 tank vol.jpg

• tlkenyon
• Oct 6, 2024

R25 Fuel Tank.jpg

• tlkenyon
• Oct 6, 2024

So now and then I stick the tank while level on the trailer confirm tank contents - often before and after adding fuel. I have also correlated the fuel depth vs gallons while floating at a 3 degree bow up attitude. Not much difference. Lotsa work, but the tank got a good cleaning and now I can be absolutely certain how much fuel I have.

Despite having lotsa measurements and such, there is no consistent under-or-over reporting from Garmin. Sometimes it is almost exact, and sometimes 10% of so off, both over and under. Seems to be most accurate at about 1600 rpm. There is slight correlation for under reporting (reported less fuel used)at lower RPM and over reporting (reported more fuel used) at higher RPM, when there is a long enough consistent run to kinda make that pretty hazy correlation. I always stick the tank before leaving on a "longer" trip.

BTW - I stick the tank using a wooden dowel inserted into one of the blank fittings on the top of the tank. it is easy so see the fuel level on the wood.

IMG_1749

• tlkenyon
• Sep 12, 2021
• 1

Fuel tank fittings

 

[S. Todd]

Because I can never measure the amount of space in the tank that has air in it (forward of the fill hose and vent that causes the fuel to burp out of the fill hose), I never select the option to mark the tank full. Instead, I always enter the amount of fuel that I just added. I often do not fill our tank depending on our cruise schedule because the boat gets better mileage if I don't carry the extra weight from carrying 145 gallons of fuel if we only need 100 for our cruise plans including reserve. When I do fill the tank, it frequently begins to burp like it is full 5 or 8 gallons earlier than I expect it to.

 

[AGLyme]

RT31cb here… purchased in June.
180 gals fuel tank.
After reading this thread I decided to fill til “burp” for the first time… and check against the gauge level - which revealed exactly 1/2 a tank. Hoped the boat would only take 90 gals.
The tank took 108.3 gals.
Glad I did it. Helpful exercise. Thanks.

 

[SJCJACK]

The tank is long and flat with the fill at the back of the tank, so if the boat is sitting stern low or high it will make a difference. As stated above always fill to the burp and then reset.

The Garmin fule indicator has always been accurate for me. 3ys and 600hrs.

Thanks for posting. I've seen numerous comments about "resetting the fuel gauge" (either the Volvo or the Garmin...) can you tell me how to perform that function? Thank you

 

[SJCJACK]

Thanks for posting. I've seen numerous comments about "resetting the fuel gauge" (either the Volvo or the Garmin...) can you tell me how to perform that function? Thank you

 

[Cutwater28GG]

Telling when the tank is full when fueling is difficult. i do this by the sound it makes as it gets full. but you have to assume +-5 gals inaccurate.
Lots of slow speed motoring is less accurate on the fuel flow compared with high flow usage. I've seen this push my Garmin estimation out by +10 gals
Therefore leaving a 15gal error margin is always advisable.

Generally though getting a full tank and using the boat at planing speed, my garmin is accurate to within 1 gal over 60 gals of usage.

 

[CaspersCruiser]

I have a 2012 R27 that I often trailer. Before putting in on a Great Loop section cruise, I carefully fill the tank to capacity at a truck stop while the boat is as level as it's going to get. Then when I refuel on the water, I use the "add fuel" utility on my Garmin 5212 chartplotter to input the exact amount of fuel added to the tank. I do not use the "fill tank" option. Using the "add fuel" technique on over 2,000 cruising miles, I have most times been able to predict within +/- two gallons how much fuel it will take when filling the tank.