another gear ?
 

We got a used rc10b4 ,my son wants to put his velineon 3500 in it I have know idea what gear we need to put in it I have 18/84 know. Any help would be appreciated? Clueless when it comes to gears

I'm not into brushless yet, but I'd assume the 3500 is considered a stock-equivalent. With stock or spec motors I run 22-24T pinions on 81 spur in my B4. 23T is the starting point, go up or down according to the track. 23T is also what the rtr version comes with.

On my stepson's B4 with a 13.5 which is 3200 or 3300kv ( I think) we gear it 78/28. And after a 7 minute race the motor and speed control are still really not very warm. Also ran his GV2 buggy 83/29 with the same results.

Read on traxxas sit its a 10 turn 3500kv

What I've seen at the track the Velineon motor isn't any faster than a 13.5 motor with a 6cell pack. I have past many of them on the straight with my 27 turn brushed motor buggy.