fragile spur gears???

Okay guys, it seems to have been fixed... Squee was right with his ratio proposal :)
I also untighted my slipper and now I can play with 100%epa... but top speed is considerably affected.
Motor temp gets down, and had to cut 2 wing of the heatsink (my model is a 540C 9L+9920) to catch the right gear mesh with the 66t spur......
>>> Do you think that I can try 18/66 without problem?
>>> Can someone give me a range of ratios to run with my BigJoes, I only have 40series 6.5"tires...(mulsher, masher, bigjoe)?