Tank size depends on your year. My '79 has a 21 gal capacity, whereas my '83 is 17 gals. Mileage will depend on a number of variables, but the worst you're likely to see is 18 mpg, so you should have plenty in the tank if you drove only sixty miles on a full tank.

I'd try starting with the gas cap off. If you get suddenly start to get fuel, then the tank vent is clearly faulty. (by the way, a bad vent with a good fuel pump can cause the tank to collapse and significantly reduce its capacity). If not, then look to the fuel lines and/or tank screen. Reversing the fuel lines should give you a diagnosis there. I don't think you'd hear suction at the fuel filter inlet if the lift pump were shot, but it's worth checking if the first two tests don't yield an explanation.

Might want to have that alternator checked out before you decide to take another long drive.

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'79 240D