Fuel tank won't fill - Advice?

I drive a 2019 Freightliner also,with 2 90 gallon tanks.The gauge never goes to full,just a little above 3/4 when filled.
When the low fuel light comes on it takes 120 gallons to fill the tanks,so I must have about 60 gallons left in the tanks.I stop for fuel every day,usaully when the gauge gets near 1/4 tank.Getting 7 to 9 mpg,depends on weight.I check fuel mileage on paper,miles and gallons used,it's a little more accurate than the dash.

 

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My 379 fuel supply lines are at the bottom of the tanks and return is at the top.

 

More than likely your gauges are not working properly like you say you should be getting better milage than that, so its both good and bad, unless you've got a major leak, or like you suggest your truck engine is sucking more juice than its supposed to, than it's just a faulty gauge so you'll have to put up with it till you can get it fixed.

 

If your tanks were full to start with and you went say 950 miles and it took 150gl to full it up that's 6.3 mpg...

 

The fuel level rods don't go all the way down to the bottom. Imagine having friction between two metals in fuel?! You have about 100 miles once light comes on. Gauges on freightliner also suck. Replace it with an aftermarket if you still have the model before the screens where adapted. The last culprit is theft. Your yard, bad truck stop area. Maybe while you showered it disappeared. get a cheap SD card surveillance camera and zip tie it under the truck and see if you have a fuel leak while driving. Under suction and pressure you might be losing fuel.

 

Tank capacity is usually stamped on them right around the filler neck. Along with a warning to only fill to 90% capacity. Ideally both should be even. Remember when fueling, they’re still equalizing. So if you’re filling one at a time, the first one is slowly filling the second one while you’re filling it too. It’s a slow process. Point being the second tank will take a bit less. The first tank will take a bit more topping off, after filling the second tank. Only way to be sure both are topped off.

 

Step 1. Start ignoring your computer mpg

Step 2. Fully fill up, mark down mileage

Step 3. Drive 500+ miles

Step 4. Fuel and calculate from miles traveled what your mpg ACTUALLY is.

Step 5. Do this for over a year

NOW you have a handle on your mpg. Track record for how it does in variable terrain, high idle, temperatures, can make some better guesses as to where doesnt have as good a quality fuel etc etc