I am using 18/81.
I was using an xcelorin speedo with a 6.5T Losi, now I've got a Novak GTB, with a Novak velocity 6.5.
Acceleration is fantastic, and top end is more than I can handle. I'm only driving for fun though, not racing
. The speedo is staying very cool.
In the manual of the Novak GTB ESC, it does mention 18/81 as a good starting point for a B4 too.