According to Sheldon Brown's Gear Calculator, wth 27" wheels and a 52/13 gear, if you are turning the cranks at 80 RPM (considered by many to be a little slow) you should be doing 41 km/h. If you are turning the cranks at 100 rpm (considered by many to be a good target cadence for most road riding) you should be doing 51 km/h.

Unless you are training for the olympics, you don't need to get a higher gear, you need to teach your legs to spin faster.