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The mass of the aspirin is $320\;{\rm mg}=3.2\times 10^{-4}\;{\rm kg}$. The energy 'equivalent' is

$$E=mc^2=(3.2\times 10^{-4}\;{\rm kg})(3\times 10^8\;{\rm m/s})^2 = 2.88\times 10^{13}\;\rm J.$$

Now, the car goes 12.75 km for every liter of gas and each liter of gas provides $3.65\times 10^7$ joules of energy. Thus the distance traveled for a given amount of energy is

$$\frac{\rm dist}{\rm energy} = \frac{12.75\;{\rm km}}{\rm L} \... ... L}{3.65\times 10^7\;{\rm J}} = 3.49\times 10^{-7}\;{\rm km/J}.$$

Thus the distance traveled when powered by an aspirin is

$$\begin{eqnarray*} {\rm dist} &=& \frac{\rm dist}{\rm energy}\cdot {\rm energy}\\... ...7}\;{\rm km/J})(2.88\times 10^{13}\;\rm J)\\ &=&10^7\;{\rm km}! \end{eqnarray*}$$

By comparison, the circumference of the earth is $4\times 10^4$ km, so that the aspirin would allow the car to go around the earth about 250 times before running out! Now that's some sweet gas mileage.