The theoretical top speeds, or at least the speed differences between gear ratios can be calculated here:
http://www.cryohold.com/bike/gears.html However, the actual top speed that can be achieved depends on the horsepower available and the combined drag coefficient of the bike and tucked-in rider. The drag force is proportional to the velocity-squared so it quadruples when speed doubles. At high speed, almost all the horsepower is needed to overcome aerodynamic drag. Because horsepower increases with increasing rpm, most bikes can reach a higher speed in 5th than in 6th gear because they can make more horsepower at the higher rpm reached in 5th, if that makes any sense. So you need to actually try each gearing combination to see which set-up will give you the highest top speed, or use a wind tunnel.
http://speedzilla.zeroforum.com/zerothread?id=43210