My textbook states that branched alkanes are thermodynamically *more stable* than straight chain alkanes. Of course, *iso*-octane has an enthalpy of formation of $\Delta H\_\mathrm f$= $\pu{-259.34 kJ mol-1}$ and *n*-octane has an enthalpy of formation of $\Delta H\_\mathrm f$= $\pu{-249.73 kJ mol-1}$ which indicates that *iso-octane is relatively more stable than n-octane at room temperature*. When they are used as fuels, both produce energy and the enthaply of combustion of *iso*-octane would be less than enthalpy of combustion of *n*-octane. So, *n*-octane should be more efficient fuel than *iso*-octane. But in petrol, *iso*-octane has more composition than *n*-octane. Why?