Quote: Just run the 3.91, and see what it does. Then if you need to switch, then go for it
what he said. Usually you'll get at least one warning before you are booted. As mentioned 75% of the time nobody will even care. But to answer your question if the car is set up for 3.91's then you'll drop .3-.5 seconds off your et as the car will not be into the powerband quick enough.
