There is a shifter kit available that should allow you to get that type of speed. However, I beg you to consider that these are bicycles and aren't designed for the type stress that those speeds put on the system. A critical frame failure at 40 mph can be extremely painful/fatal! If you want to run with the big dogs get a motorcycle!
If it can't be broken I'll find a way!
I'm not lost, I'm exploring!