Originally Posted by b4maz

What kind of batteries are you using? how many C? like someone said before, your voltage drop is pretty big. if you're using low quality batteries your voltage under load will drop significantly.

When I do the calculations, it comes out to exactly what you're testing:

4s: 39.94 mph
6s: 60.33 mph

I think you're inputting something wrong into the speed calculator. This is what you should be getting:

4s:
Differential Ratio: 4.3
Transmission Ratio: 1
Other Ratio: 1
Spur Tooth Count: 46
Pinion Tooth Count: 20
Total Voltage: 14.8
Motor KV: 2200
Tire Diameter (inches): 4
Tire Ballooning (inches): .125
Motor Current Draw: 100
Motor Coil Resistance: 0.006
Spur/Pinion Ratio: 2.3 : 1
Total Ratio: 9.89 : 1
Tire Circumference (inches): 13.35 inches (339.13 mm)
Rollout: 1.35:1
Total Motor Speed: 31240 RPM
Vehicle Speed: 39.94 mph (64.15 km/h)
Effective KV Value: 2110.81
KT constant: 0.61 oz-in/A
Motor Torque: 0.32 ft-lbs
Final Torque: 3.17 ft-lbs
Final Power: 1480 watts (2 HP)

6s:
Differential Ratio: 4.3
Transmission Ratio: 1
Other Ratio: 1
Spur Tooth Count: 46
Pinion Tooth Count: 20
Total Voltage: 22.2
Motor KV: 2200
Tire Diameter (inches): 4
Tire Ballooning (inches): .125
Motor Current Draw: 100
Motor Coil Resistance: 0.006
Spur/Pinion Ratio: 2.3 : 1
Total Ratio: 9.89 : 1
Tire Circumference (inches): 13.35 inches (339.13 mm)
Rollout: 1.35:1
Total Motor Speed: 47520 RPM
Vehicle Speed: 60.75 mph (97.59 km/h)
Effective KV Value: 2140.54
KT constant: 0.61 oz-in/A
Motor Torque: 0.32 ft-lbs
Final Torque: 3.17 ft-lbs
Final Power: 2220 watts (3 HP)