It may be an old thread, but I'm going to give this a try !
If I only fill up to half-tank, that means having 26L (diesel) or 21.8 kg less weight on the car.
That's about the combined weight improvement from dumping the spare and my own weight loss
There are some more potential weight gains as well, so I'm going to combine all of them and see what gives.
My favourite fuel station is right on my commute - a detour of only 25m (or less than 30 yards) and 1 extra stop.
Also, my car has no issues when the tanks level drops - when it gets really low (about 3L from empty) the engine performance will automatically be reduced to conserve gas.
Even when run dry, the fuelsystem will refill itself when fuel is added to the tank.