Grifter, motorcycle.com compared the Ninja 250, Nighthawk, and Virago. Here is the torque curves for those bikes. For the Ninja, there is a nice jump in power from 4,500 rpm up to 9,000. I'm sure you notice this on your bike. So while street cruising, you want to keep the rpm's around 4,500. That will give you instant acceleration. At 45 mph in sixth, you were probably well below 4,500.