tuning for economy

Hi,

If i target 15.0 or 15.5 , Do i need to retard the timing at the same area where i target 15.0 or 15.5 ?

Also do i need to retard the timing 3 or 2 degree when switching from 95 Oct to 91 , even if the dealer of the car recommended to use 91 Oct ?

Flame speed decreases as lambda moves away from 0.87 in either direction, if you are not knock or pre-ignition limited you will get best efficiency advancing timing to match the lower flame speed. This may only be valuable at low load as you may run into knock or pre-ignition if excess fuel is required for thermal control/combustion stability.