ACDelco recommends the right balance of Cold Cranking Amps (CCA) and Reserve Capacity (RC). Select a battery that has the recommended amount of CCA, found in your Owner’s Manual, and with as much RC as you can get. A battery with a higher-than-recommended CCA will not start your car faster. This is due to gear reduction starters, lower viscosity oils, fuel injection, etc. The average car cranks no more than 3 seconds and as a result does not need more CCA than what the vehicle manufacturer recommends.
RC, however, is becoming more important due to the amount of vehicle electronics. The more RC you can get, the better your battery will perform.