Another slight diversion from the core topic of this thread, but in a part-answer to why aircraft fuel gauges are inaccurate compared to car gauges, it has something to do with the way the tanks are shaped. Most car tanks are quite "deep", so a small fuel usage results in a large drop in fuel level in the tank. Aircraft tanks, because they are often in the wing, are necessarily "flat" in design, so even a large difference in fuel quantity might be only a few millimetres of level difference. Measuring the level of a fluid sloshing around inside a tank accurately to a millimetre or so is quite complex, hence the gauges (built for cheapness) are often quite inaccurate.
It is interesting to compare the Cessna 172 fuel gauges with the Diamond Katana - the former has "wing tanks" while the latter has a "fuselage tank". I have never know the Katana gauge to be wrong by more than about 5 litres (and even that is a lot in a 76 litre tank) yet the Cessna can be out by 50 litres (out of 230 - the Cessna I'm used to has the long-range tank option).
Modern aircraft use more advance techniques to measure the fuel quantity (e.g. fuel flow, capacitance meters etc) and as a result the systems cost more. They do get reasonable accuracy even on wing-tanks (e.g. the Diamond Star which is accurate to +/-5 litres in 150 litres in my experience). But it does cost more. It is a major mod to fit a new type of sensor/gauge to an old aircraft as far as the CAA are concerned. It's not going to happen, guys and gals.
As to why some flying schools ask for aeroplanes to be returned near-empty when possible, well, there is this little detail of Maximum Takeoff Weight. You might be flying a four seater solo, but the next pilot might have three passengers and a lot of baggage. Full tanks may not be a legal option for them. Would you want to make an over-weight take-off? Even if the plane makes the take-off and crashes on landing at a safe weight, the insurance company is going to be rubbing it's hands with glee while the club wrings its hands in horror.
Now back to the regularly scheduled mud-slinging,
MD.