whats the highest gearing I should use?

[suicideneil]

Differential Ratio: 2.8461538461538462
Transmission Ratio: 1.8333333333333332
Other Ratio: 1
Spur Tooth Count: 68
Pinion Tooth Count: 18
Total Voltage: 14.8
Motor KV: 2200
Tire Diameter (inches): 5.9
Tire Ballooning (inches): 0
Motor Current Draw: 0
Motor coil Ω: 0
Spur/Pinion Ratio: 3.78 : 1
Total Ratio: 19.71225 : 1
Tire Circumference (inches): 18.54 inches (470.8 mm)
Total Motor Speed: 32560 RPM
Vehicle Speed: 28.99 mph (46.57 km/h)
Effective KV Value: 2200

29mph, thats really shite given your setup's potential (you have good lipos I assume?)

68/22 = 35mph, much better, but still a little slow.

68/24 = 39mph, much more like it.

If you bumped the voltage upto 5s or 6s, then speeds around 45mph and 50mph would be easily do-able, given good batts.

Just keep an eye on your temps:

180 for the mtor, ideally 140-160.
160 for the esc, ideally 120-140.
120 for the lipos, ideally 100-110.