What prop were you using? The point of gearing down is to turn a larger prop for more thrust. If you put the stock 8x4 prop back on, total power would be diminished by the ratio of the gearbox.

These 3" wheels... Did you get light foamie wheels, or regular R/C wheels? Regular 3" wheels are HEA-VY. Probably added 1/2 pound to the weight of the airplane like that.

What kind of surface are you trying to ROG from? Heavy, thick, tall grass? ****eddaboudit.

Forget about a 3S LiPoly too. If the battery will handle the Amps, the motor won't handle the Volts. It'll sling a winding or burn out the brushes quickly. I highly suspect that your 3S LiPoly is off a park flyer anyway, and is nowhere near robust enough to take the roughly 30 Amps this motor should be drawing.

According to P-Calc: 8 Cell 2000mAh NiCD, Magnetic Mayhem, 2.5:1 gear ratio, 12x8 prop should draw about 28 Amps . That should be plenty to haul a PT Electric off short-cropped grass if the plane doesn't weigh too much.