Need help with my G3-build

[suicideneil]

1. The pro gears are truggy ratio, which means you would have to use a much larger pinion/ smaller spur than normal to get the desired gear ratio for any given speed; normal buggy ratio non-pro diffs only need a pinion 1 tooth larger than compared to the normal emaxx diffs. Get the non-pro ones.

2.

Differential Ratio: 3.3076923076923075
Transmission Ratio: 1.7222222222222223
Other Ratio: 1
Spur Tooth Count: 66
Pinion Tooth Count: 20
Total Voltage: 18.5
Motor KV: 2300
Tire Diameter (inches): 5.75
Tire Ballooning (inches): 0
Motor Current Draw: 0
Motor coil Ω: 0
Spur/Pinion Ratio: 3.3 : 1
Total Ratio: 18.79872 : 1
Tire Circumference (inches): 18.06 inches (458.83 mm)
Total Motor Speed: 42550 RPM
Vehicle Speed: 38.72 mph (62.2 km/h)
Effective KV Value: 2300

Assuming the BM is ~2300kv, been a while since I looked it up. Think its more like 2600kv, but that would give you ~43mph on the same gearing, which is pretty reasonable for that setup- 40mph is plenty for bashing or racing really (you didnt specifcy what you would be doing with the truck).

http://www.scriptasylum.com/rc_speed/_top_speed.html most useful tool ever, thanks to BrianG.

3. Tough call- I use 2x5s in parallel on my rig for extra runtime and less stress on the lipos (Zippies, love em to bits but a safety margin is always good with cheaper lipos, regardless of brand). If you went for smaller 5s lipos, say ~3700mah or so, then 2 in parallel would be perfectly fine,- still half the weight of nimhs with much more power output and runtime. A single 2s and 3s in series is fine too, and useful for use in other vehicles, just be sure to get ones of the same specs (just different voltages obviously).
Anything able to output over 100amps is fine (e.g. 5000mah 20c), but for a hungrier motor like the BM, over 120amps is better (e.g. 5000mah 25c).

Look into the stuff Mike sells as his own branded lipos (RC-Monster), or anything from rclipos.com (Enerland based cells).

Just dont fall into the maxamps trap, or we'll disown you...

Welcome to the forum.