It's a formula based on several variables. Correction: volumetric efficiency, not "vehicle efficiency" as I initially stated. See this link for details: http://www.installuniversity.com/ins...n_9.012000.htm
Regardless, I found the Torque app can calculate VE on its own, so nevermind. Right now my Insight is in the body shop, but the rental I have (Altima) has awful VE if the Torque app is accurate...its topping out at 70%.