Compressed air is "made" by taking air into a machine and discharging the same air, at a higher pressure, from the same machine. There are two efficiencies to consider and these depend on the type of compressor. The first is the volumetric efficiency and the second is the isentropic efficiency.
The volumetric efficiency is the volume of air delivered at NTP over the volume taken in at the same NTP and may be as high as 0.85 or 85% with some types of machine. The losses are leakages past valves and piston rings, for example.
The isentropic efficiency is the theoretical amount of energy required to compress the gas over the amount of energy practically required to compress it. The lost energy represent that required to overcome the friction of the machine, the friction and turbulence of the gas within the machine, its filters, pipework etc. and, above all, lost as heat. Unfortunately for you, three guys called Mr Boyle, Mr Charles and Mr Gay-Lussac (look them up in an encyclopaedia, if you have not already met them) developed a series of physical laws, bearing their names. They stated that pressure, volume and absolute temperature of a perfect gas are all interdependent. That means that if the pressure goes up, so does the temperature and the volume goes down, or a combination of them. In a well-designed reciprocating compressor, you can reckon on the heat loss representing about 0.3 of the energy input and the overall isentropic efficiency will be typically 0.6.
The overall efficiency may therefore be 0.85 x 0.6 = 0.51, with a reciprocating single stage compressor. Unfortunately, you talk about 300 atmospheres, which is really beyond the scope of a single stage compressor. You will require a three-stage compressor with additional frictional losses, so let's say an efficiency of 0.4, which would be typical.
OK, you have your compressed air: what are you going to do with it. Push it into a motor. Now, Messrs Boyle, Charles and Gay-Lussac have a say here, as well. You are reducing the pressure of your gas, so the temperature will drop and so will the volume at any given pressure and the efficiency of the motor will drop, because it will not be able to absorb sufficient heat to make up the loss fast enough, so the air coming from your exhaust pipe will be frigid. So sad! The efficiency will probably be similar, say, 0.4, so the overall efficiency of your system is now 0.4 x 0.4 = 0.16 or 16%.
Now all this assumes a perfect gas. Dry air is such, but natural air is not, because it contains a gas which really upsets the applecart: water vapour. Even in the middle of the desert, the relative humidity can be 20 or 25%. When you compress air, the relative humidity rises to well over 100% and condensation occurs: yes, your air tanks will start filling with water unless you take measures to prevent it. Several ways can be used but the most efficient is a cryogenic separator, which chills the air to, say, -50°C, allowing the compressed air and most of the moisture to separate. Guess what? Separators require energy!
Bang goes your efficiency again.
Now, what kind of compressor do you need? I've talked of reciprocating, which is the most usual, but, of the positive displacement types, you can also have sliding vane, Lysholm, plus centrifugal turboblowers and axial turboblowers for non positive displacement types. Each has its advantages and disadvantages. Please make your choice, because your design will depend on your choice,
So, with an overall efficiency not exceeding about 10% your air car is going to be a huge success, isn't it? This is not negativism it is pure science and engineering, two subjects about which you are blissfully unaware. You break our monitors with your total bullshit, which is derived from your ignorance about the most basic physics.
So, please take a word of simple advice. Shut up!
