How heavy is your truggy?
If it is ~ 1.5 to 2 lbs, then 540 motor is enough
If it is > 2lbs, then 550 motor is better.
Also,540 motor and 550 motor shaft are different size. 540 usually is ~ 3mm while 550 motor is ~ 5mm. This depends on what pitch size you need for your car's spur gear. (ie 48 dpi or mod 1)
550 motor is always more suitable for heavier cars because they can give you more torque.
If 2S, choose a motor in the 4000kv range
If 3S, choose a motor in the 3000kv range
If 4S, choose a motor in the 2000kv range
In general, it is better to run higher cell with a lower kv motor. this way, all electronics will be cooler.