tink, 6.6:1 is "D" gearing. You're on the right track, but you really don't need a spinner. In fact if you can find one of the hardened short shafts which won't allow a spinner you'll be better off. One weak point on the GWS gearboxes when used in foamies is the soft ***** shafts. A hardened short shaft will only set you back a couple of bucks. Money well spent. All E RC is one place that sells the shafts. You'll need a 3mm.