I shift at 3000-4000rpm in daily driving and 4500+ when doing more "spirited" driving. When cruising, I usually stay around 3000rpm, but to accelerate and pass or change lanes I'll usually downshift to get into the 3500-4500 rpm range to have better throttle response. I can't imagine the sound your car would make at 40mph in 5th gear, but I wouldn't do it. I use 4th gear up until about 65-70mph, so 5th should probably only be used for highway cruising above 60-65mph.