FDR stands for "final drive ratio"
It is the amount of teeth on the spur gear divided by amount on the pinion multiplied by the internal transmission ratio.
( ----- ) * internal = final drive ratio
This is simply how many rotations the motor goes per rotation of the axle.
An FDR of 4.0 means 4 motor revolutions to 1 axle revolution.
If you don't know your vehicle's internal drive ratio, divide the amount of teeth on the differential gear/pulley by the amount of teeth on the drive gear/pulley.
If your brushed motor is older, a nicely setup 17.5 (geared properly) should probably be much faster than the 27t with alot more torque and more durability.
If all else fails, gear the motor up and watch temperature after a run.
If you can't hold your finger on the motor, its too hot and the timing or the gearing must be decreased. This gearing change should be drastic (a three to five tooth increase) from the stock pinion.