are u sure the fuel pickup/ gauge float are installed properly ?
the whole assembly might be misaligned such that it is not in the lowest possible position. this would cause your gas gauge to read wrong and also you will run out of fuel even though the tank is not empty.
if you remove the carpeted panel directly behind the seats you will see a round steel plate where the fuel lines and wires go into the tank. there should be an alignment mark on the plate that should align with the seam in the fuel tank.
if it doesn't get a strap wrench and loosen the big plastic ring, align the plate and tighten it up.

if its aligned properly you probably have a wrong fuel gauge and prepump problems

if you are really eager you can run your tank almost empty and siphon the rest out and measure it or look inside the tank by pulling the pump.
oh yeah, don't smoke while you do it ;)

btw the most i ever got out of my tank was 48L. if you get that low the car hesitates beacuse the fuel gushes and the pump takes in air
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Canadian 1988 745 320k km B230F LH 2.2 EZ-117K AW70 Bendix/ATE no ABS

When I bought my 1990 745 18 months ago it seemed my gas gauge was wildly inaccurate. When I filled the tank the needle would peg the top and then I filled it after the gauge hit the red zone it took only ~8 gallons or ~30 liters. I knew by my records I was getting 25mpg, so I wasn't worried (lots of gas left in the tank I thought) and I knew about inaccurate Volvo gas gauges, since this was my second Volvo. One day while driving with what I thought was a 1/2 full tank (needle in the red zone) the wagon hesitated and stumbled while merging on a Interstate on ramp in heavy traffic at high speed. Very scary. I managed to drive the car to a gas station and my fill was only 9.5 gallons, 36L. Two minor annoyances that I had notice about this car now made sence. 1) occasionally in a tight turn with a low tank the car would slightly hesitate and 2) occasionally while turning I could hear a very high pitched whine, a wheel bearing or differential bearing I thought. Wrong, the car was hesitating because the in tank fuel pump was sucking air and the fuel pump was whining because it wasn't lubricated by the gasoline and was on its way to failure.

Several weeks later the fuel pump did fail and I replaced it myself. (For clarification the wagon has the Rex/Regina system. There is only one fuel pump and it is in the tank) I found that a PO had work done to the fuel pump previously (the wiring harness had been cut and crimped inside and outside the tank) This is the only Rex/Regina I've worked on and I didn't have a stock system to compare it with, but I suspect that the fuel pump/sender assembly was replaced at some time with an assembly from the junkyard and came from a different model year, a 740 sedan, or maybe a 760. **I don't think the pickup reaches the bottom of the tank and I think the sender is designed for a differently shaped tank.** I've had zero problems with the pump I put in and I never let the gas gauge inter the red zone before filling.

My S60 holds about 18 US gallons which is about 68 liters.
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George Downs Bartlesville, Heart of the USA!

Yes, really; 60 liters.

Have you measure how much it took to fill after running dry? I've found that there is quite a bit of fuel when both my cars (one has the 80 litre tank) read in the reserve. The 60 litre car has taken 55L (14+ gal) without running out.

If you have the older type metal tank, it could be crushed and therefore smaller.