Hey Tom Erik
I am running my Lazer with 78 spur and 17t pinion. I think it depends very much on which track you're racing on. During the race in kodal a month ago it was very hot weather, and I noticed that the motor and esc getting quite hot.
I am running a Reedy 3star 6.5 in mine, and I think running a 78/16 gearing I still will have enough speed.