need help with converting a hyper 7

[lincpimp]

Use BrianG's speed calc.

http://scriptasylum.com/rc_speed/_top_speed.html

Your results should look something like this:

Differential Ratio: 3.3076923076923075
Transmission Ratio: 1
Other Ratio: 1
Spur Tooth Count: 46
Pinion Tooth Count: 9
Total Voltage: 14.8
Motor KV: 5000
Tire Diameter (inches): 4
Tire Ballooning (inches): 0
Motor Current Draw: 0
Motor coil Ω: 0
Spur/Pinion Ratio: 5.11 : 1
Total Ratio: 16.90598 : 1
Tire Circumference (inches): 12.57 inches (319.19 mm)
Total Motor Speed: 74000 RPM
Vehicle Speed: 52.09 mph (83.67 km/h)

An 8 tooth pinion looks like the best bet, however you may have some issues getting it to fit physically. Smaller pinions usually cause problems, as the motor can only get so close to the spur before it contacts with the diff output. The smallest hardened pinion Mike carries is a 9 tooth. I would give that a try... I usually try to gear for around 45mph, the 9 tooth will give around 50mph with that motor.