Hi Michel,
Here's a simple to use calculator that you can play around with to see what the RPM's will be at various speeds with various rear gears. My kit from ERA is in the final stages, and I have no experience driving different configurations. I went with their recommendation - 3:54 rear with .64 5th gear in a TKO-600. This is thought to be the "best of both worlds", providing fairly low gearing with reasonable RPM's on the highway. Gearing is a topic where you'll get lots of different opinions.
Gear Calculator
Regards,
Kevin