First question is what is "full"? Is it when the pump clicks off? If so then you probably are at least a gallon under filled since diesel foams quite a bit and if you are patient you can squeeze more fuel into it.

I'd say if you fill to the neck with liquid and are only getting 17 gallons into it from the point where the needle is at 0 then the gauge or sender is off. Typically if I drove it to 0, which I have done, I would put around 20 gallons in. I always fill to where liquid (not foam) is at the neck, usually about a gallon past the point where the pump clicks-off automatically. I typically fill when the reserve light comes on, and put a little over 18 gallons in.

Even if your gauge is off I would not fret...if you have a little extra in the tank you'll be assured of not running out without having to think about it.

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Marty D.

2013 C300 4Matic
1984 BMW 733i
2013 Lincoln MKz