Don't lug the engine that way. But, yes. 150 mph is about 6,000 rpm. Don't ask me how I know! So, at 50 you'll be turning 2,000 rpm. At speeds below 60, you should be cruising no higher than 5th. Both are overdrive gears. It is better to keep cruising rpm above 2,500 rpm so that if you need to accelerate, you are not lugging the engine and can access some boost.
