http://scriptasylum.com/rc_speed/_top_speed.html
That's the top speed calculator which ought to help you. I have found that this seems to be off a little bit but a 22t should put you about where you want speed wise but I would suggest going to an 8xl and gearing higher.
You can copy and paste the information below to post your values into a forum thread:
Differential Ratio: 2.8461538461538462
Transmission Ratio: 1.8333333333333334
Other Ratio: 1
Spur Tooth Count: 68
Pinion Tooth Count: 22
Total Voltage: 14.8
Motor KV: 2382
Tire Diameter (inches): 5.75
Tire Ballooning (inches): 0.5
Motor Current Draw: 0
Motor coil Ω: 0.0056
Spur/Pinion Ratio: 3.09 : 1
Total Ratio: 16.12821 : 1
Tire Circumference (inches): 21.21 inches (538.63 mm)
Total Motor Speed: 35253.6 RPM
Vehicle Speed: 43.89 mph (70.51 km/h)
Effective Speed Rate: 2.97 mph/V (4.76 kmh/V)
Estimated Adjusted Speed: 41 mph (66 km/h) - 6% loss
Effective KV Value: 2382
Motor Torque: Amperage not specified...
Final Torque: Amperage not specified...
Final Power: Amperage not specified...