The fuel tank is metric which converts out to 21.1 US gal (80 litres). Out of that 2.6gal. is considered the reserve (when your needle hits the red or white margin on your gauge).
I have pressed my luck once or twice and ran it almost on air but only was able to squeeze 19.8 gal into it. Its tough because of the foaming of the fuel especially with winter diesel. I get the most complete fills when the current car sits nose down at an angle....it seems to allow the trapped air in the foam to move out the neck easier and allow more tank volume for fuel. On some of my diesels it was the opposite.....nose up/ tail down vented air out better.
Truefully...I try to not allow the tank to drop bellow the 1/4 mark. Priming and restarting the thing out on a busy highway is not in my future...I've done that before...not fun! Plus you stand a chance of sucking water/ wet fuel out of near empty tank.

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FRED

Daily Driver: 98 E300TD 199K
Hobby Car: 69 Austin Mini
Past Diesels: 84 300SD, 312K
87 300SDL, 251K
94 Chev. K-1500 6.5Ltr.TD, 373K