The gearing thing on the Durga is always fun
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I ran an LRP Vector 5.5 which they recommend is run at 13:1 on high grip tracks and 14:1 on low grip. According to the helpful oOple gear chart, on a 91/37 spur/dif combination there is no exact match but a 14 or 13 tooth pinion should get the job done. It didn't for me, I ended up with a motor too hot to go near let alone touch. I had started on a 16t originally and worked my way down. To be honest I may have an iffy motor. I've given up with it now and gone to a 12 x 2 brushed motor and I'll see how that works out.