Put it in D4 unless you're climbing a steep grade or something, then a lower gear (D3) might be necessary.If you're missing your stick you can still pretend by shifting through the gears, but when the transmission is ready to shift, it'll do it on its own.The difference between D4 and D3 is that your vehicle probably has some sort of overdrive system on it, meaning that you'll get better gas mileage in D4 (overdrive) than D3.