MGM controller do I go for amperage or cell count??

[Finnster]

One other thing. To get #'s that match my radar, I typically use ~7" on the tires. The ballooning def has an effect, and kv values are not exact. As you go faster, more balloon is needed.

Otherwise I use 3.5v/cell lipo, or 2.8V/cell A123. These are known, measured values under load. If they are quite alot higher or lower, you prolly don't have a properly sized batt for your app.

EX:
Differential Ratio: 2.8461538461538462
Transmission Ratio: 2.2941176470588233
Other Ratio: 1
Spur Tooth Count: 40
Pinion Tooth Count: 16
Total Voltage: 14 (either 5S A123 or 4S lipo, calcs the same)
Motor KV: 2084
Tire Diameter (inches): 7
Spur/Pinion Ratio: 2.5 : 1
Total Ratio: 16.32353 : 1
Tire Circumference (inches): 21.99 inches (558.58mm)
Total Motor Speed: 29384.4 RPM
Vehicle Speed: 37.49 MPH (60.22km/h)
Effective Speed Rate: 2.66mph/V (4.27kmh/V)
Effective KV Value: 2084

Before I taped the tires I got 40-41mph reliably (more balloon) now its actually a bit slower @ 38mph on radar, but is much more controllable. IMO, less than 6" for normal maxx tires will under est the top spd. These are w/ mashers, so YMMV.