Driveability index is a measure of a gasoline's performance across a number of operating conditions, covering cold start and warmed up operations. It incorporates elements of the distillation profile for the gasoline (temperatures for evaporation percentages, such as T50) as well as ethanol content.
The driveability index is calculated as:
DI (in degrees C) = 1.5x(T10) + 3.0x(T50) + (T90) + 1.33x(Vol % ethanol)
DI (in degrees F) = 1.5x(T10) + 3.0x(T50) + (T90) + 2.4x(Vol % ethanol)
Where T10, T50 and T90 are the temperatures at which 10%, 50% and 90% of volume boils off, respectively.