An intelligent system could take more advantage of an increase of knowledge than a dumb one, so I should propose a simple formula: the derivative of efficiency

An intelligent system could take more advantage of an increase of knowledge than a dumb one, so I should propose a simple formula: the derivative of efficiency with respect to knowledge is proportional to intelligence. $$ I = \frac{partial E}{partial K} \simeq \frac{\delta E}{\delta K} $$ In order to estimate $I$ you have to consider that efficiency and knowledge are task related, so you could take some weighted mean $sum_T C(E,K,T)*I(E,K,T)$ where $T$ is task category. I am thinking in $C(E,K,T)$ as something similar to thermal capacity or electrical resistance, the equivalent concept when applied to task. An intelligent agent in a medium of low resistance should fly while a dumb one would still crawl.