Usually you want to set the final drive ratio so that the engine just touches the rev-limiter (or redline in older cars) at the end of the longest straight. Zerobandwidth's tweak to Kunos' 488 GT3 (the tweak is available here on RD but you need the Kunos dlc to make it work) seems to be the exception. If you gear the car long (so that, theoretically, it could reach 180 mph), and the car actually tops out at 155 mph; normally you would go back to your setup and set the theoretical...
zbw's Ferrari 488 GTE (aka GTLM) @ Spa Francorchamps
zbw's Ferrari 488 GTE (aka GTLM) @ Spa Francorchamps