Hi, the ideal one is 3.93 for fuel economy, acceleration and top speed. With my current 3.46 final drive, my 1st gear max speed at 6.5k rpm is close to 90 km/h, this actually cost me a bit of fuel but engine is super low speed for cruising on highway at 120k.You're on auto right? Did you find out the differential's ratio that's suitable for your current engine/gb?